And every checking runs a whole CI run?

We had it internally with our teams that open a PR to then push like 10-20 more commits but never actually interested in the client builds etc. turned out they opened the PR as a checkmark/ way to share the current state. We set cooldowns and auto cancel for the ci. And then there is the developer who uses the CI compute to run tests instead of running them locally for various reasons. We had to remind that compute isn’t for free.

This is too generous, they aren't even filling in the holes.

I suspect there is a cacophony of work that happens when a commit hits the server. That request needs to get replicated, git repositories need to be repacked, pull requests need to calculate diffs, CI jobs need to execute, on and on.

That's also just assuming the good-faith usage. There are probably plenty of adversarial and poorly behaved scrapers that are putting additional load on the system.

I'm not sure if it was entirely true, but there are stories that after Microsoft bought Hotmail in the mid-90's, they quickly attempted to move them from FreeBSD (?) to Windows NT. But it failed miserably, and they went back to the original stack for another ~decade.