That's the number it shows since the release. It's likely the number was approximated from the original number of simultaneous players. It doesn't (and can't if it's really based on this data) include any sales after release. Not to mention the whole approximation is not really precise, which SteamDB states clearly with their wide margin of error.
Another thing is that we don't know size of non-Steam sales. As I wrote before, there were at least 2 hints already that for Civ7 Steam holds less than half of total sales.
It's 50% of those who left the review (something 20K+), not of those who buy the game. It's not a random sample as people need strong stimulus to leave a review (or write on forums), so it's hard to say how many people of those who bought the game actually like it. Could be anything between around 20-80%.
DLC not only sales itself, it also sales base game, especially if timed with discounts, which it usually does. Another thing is that many people already purchased this DLC as part of founder's edition.
All in all, I found the data pretty vague to estimate anything. Total Civ7 sales at the moment could be anywhere between 1-2 millions (we probably have cap at 3). With summer sale, if summer patch will be good, it could have another half a million base game sales (wild estimation with huge error margin, of course). Together with founder's edition, the DLC could have up to million units sold by the end of summer.