This is true. A hundred full-time QA testers working 40 hours a week for three months (13 weeks) straight rack up a total of 52 000 hours of playtesting. A million users playing for thirty minutes each rack up a total of 500 000 hours of gameplay.
You could devote insane resources to QA testing and in just thirty minutes from launch, players have eclipsed the total testing by an order of magnitude.
However - and this is where the issue lies - once the game is out, and all the feedback is coming in, why does it still take a lot of time to adjust things? Why do they not have one person who spends one afternoon, say 1 pm to 5 pm, first three hours skimming through forum posts, reddit posts, steam discussions et cetera that are about game balance (who's weak, who's strong, etc etc) to make notes, and then in the last hour of their afternoon gets into the XML files and tweaks some numbers here and there to better balance civs and leaders? Just halve Hawaii's naval culture bonuses, reduce the values of the Maya bonuses, et cetera. You can quite literally do a rough balancing pass like that in one afternoon with near-zero chance of introducing bugs. I've done such passes myself in mods for Civ VI, Civ IV and Colonization. And Civ VII actually (I should upload that mod - only game pacing though).
Obviously, bugs can be quite a bit harder to track down, but even then, some of the reported bugs are this simple. A building's yields and tooltip not aligning can only be in two spots, for example - the tooltip or the building's XML.
There's 2 big issues with the second part, and almost certainly because it's a big company with processes. Speaking as someone who has worked in multiple sides of these roles...
First, for random bug fixes, sure, as a user it might take you 5-10 minutes to find the xml line that's wrong, update it, and reload your game. But for a QA resource, it takes them a few extra minutes at least to read through the bug report and figure out what's wrong. They probably have to confer with a dev to check what the intended behavior is (ie. is it a bug in the text, or a bug in the ability). Then they have to go in, re-create the game in that state, and confirm that it is an issue.
Once they do that, they probably package that up to the dev who was responsible, who has to more or less repeat all those steps, and do the fix. And then probably they have to wait for the next day to get it in a build, where it can go back to the QA person to re-verify that it has actually been corrected. Never mind any part of that process that needs to double-check that nothing else broke - other civs with similar traditions, does the bonus still work properly on regular/coastal/navigable rivers/regular rivers/etc... as necessary? Even for a simple bug, it's far from a simple process to fix.
Second, for balance stuff like Hawaii, Maya, etc... that's really up to the dev/balancing/progression developers. Sure, I can look and see that Maya's bonus is like super OP later in the game, but if you drop it to 5% or 10% from 15%, does it become too weak early? How does that impact the rest of the Mayan abilities? Or do you even want to do that - I mean, at some level, if everyone is OP, then nobody is. Never mind I would imagine that a lot of those teams are still fighting the regular bug reports, it can take some time to figure out the balance issues. Plus often you have multiple teams and people at play.
They did a balance pass on a lot of the mementos in this update. I'm sure one of the next patches we'll see a big balance pass over a bunch of civs, another patch will have a big balance pass over the leaders. And it might also have a little company culture at play too. Some games and companies are happy to constantly be updating things - maybe this patch they drop Maya from 15% to 5%, and then next patch they see that was too much, so instead they change that to give you the city's science as production for a turn instead. Oh but oops that bonus was bugged because there's something else that gives a city a one-time science bonus and if you chain them together now accidentally you gave a city 3000 production. So you fix that to only go off the base science. And then people don't like that, so they go back to based on tech costs, but they change back to 15% but only for masteries. But that's not enough, so they change it to 15% but only for the base tech and not masteries. And on and on.... Which way is better? I don't know. Doing a lot of fine tuning you probably eventually reach a good place, but it can be frustrating for users to constantly have functionality and bonuses changing around on them.
I mean yeah, in so many cases they could probably get in a quick change without too too much issue, and without worrying about all that. But at the same time, they probably also like keeping some slightly unbalanced items in. Like sure, the min-maxers see the Maya as crazy OP when you chain everything in such a way. But the average user is probably like "oh that's cool I just finished a wonder because their ability kicked in. This game is awesome". I mean, I played as Maya, and sure, their ability was pretty crazy, but due to me not realizing as well as a few other things, I only had 2 cities that I got in before the era kicked over, and it was good, but I don't think it was necessarily game-breaking even. To someone else who really maxed them out, maybe picking a different leader or with a different setup, absolutely crazy broken setup and is demanding a balance.