I feel like that's not how you measure a game engine usage, the large majority probably don't install Godot via Steam, just looking at the numbers it's a very small sample which might not represent game devs in general
Eh, I feel like the sampling is clearly biased toward those who would install a game engine through a service that auto-updates it. (Novices and hobbyists.)
It might mean something though, FFXIV is a classic example of a game that almost nobody plays on Steam, but its Steam charts line up somewhat well with the game's increasing popularity especially with Shadowbringers and Endwalker. Of course you have to look at actual data to back that up, but soemtimes it can show trends.