I dont know about this. Either costs, production and gold, will increase with game speed as well. Mali can still buy units/buildings/etc of. They wont earn more gold per turn though while still having to pay the higher price so you will take more turns here as well. Also Babylon can still speed through the tech tree but will have to pay even more gold/spend more production on these advanced units than on normal speed (for example 250 gold per crossbow upgrade becomes 375 per upgrade on epic).
The real difference is that you can use your units more often so you have an easier time at war (more movement and combat per era). So I guess everyone with war focus will have the advantage on epic speed like scythia or sumer who get more turns for their rushes.
Sure, the cost scales, but the advantage it brings scales too. The hig advantage that Mali has (and Portugal) is thst they can buy things instantly. Sure, how much it costs also scales to be worse, and that's fine, no different to the cost in production that other civs have to deal with, but what is advantageous, the ability to buy when and where you need it and not x amount of turns ahead, is all that much the better. Mali's advantage isn't that gold is an easier resource to obtain, but that gold gives you instant buys. It may not mean more units overall, but having it on demand rather than trying to predict ahead is his strength. Plus it's gold, so he can support more units than others anyway.
Babylon has slightly different logic. Killing a unit with a slinger might save a normal civ (say) 4 turns on normal speed due to the Eureka and let's say 8 turns on slow. For Babylon, it saves 10 turns (so their ability saves 6 compared to a normal civ) on normal, while they save 20 turns (so their ability saves them 12). That better increase means alot when war is being conducted at normal pace. While everyone else is struggling to research, you're getting entire free techs, which often is not constrained by speed.
Let's take a practical (but simplified) example, Babylon and England both found their first city on coast. Free sailing for Babylon and 40% for England.
Saves Babylon say 13 turns on normal or 26 on slow. Let's say that they then send a Builder to explore across the water. So, founding their city on the coast saves them an additional 13 turns.
England does the exact same thing. On the normal speed, theyd save 40% so have to spend 7 turns researching and then can send that builder, so on normal speed, Babylon gets a 7 turn head start. Not bad. On slow though, that's 14 turns. The builder can uncover a lot of distance in 14 turns, which means the advantage Babylon gained from their ability in that instance was double what it would have been on normal speed.
That's obviously highly contrived to make it simple, but the point is there: instant techs are more valuable on slow speeds than high ones.
The ability scales positively with slower speed, and becomes better as you slow things down.