I would say yes it’s like a tiny database but as well as all the other things your added. And I think that’s a good thing, because it does this at the type level!
I’m actually seeing if I can build a parser using bevy_ecs just because the way their state machine works, it looks like it would be fun
In this case systems are the S of ECS and contain all your game logic, acting upon the entities and components (E and C).
By system management I assume they mean the APIs bevy offers for scheduling and sequencing systems and events so your game logic remains modularized while still running in the correct order.
It seems like you're calling an entire game engine "ECS". I'm not sure what the point is here, the whole question was what justifies having a 50 MB hello world binary. It doesn't matter what you put into a data structure, the data structure itself shouldn't make 50 MB binaries.