Did they also warm up caches before open?

It also ensures you can come back up after an unexpected shutdown.

This is the best reason. Knowing your business systems will simply come back with the power, and having that tested on a daily basis will quickly put everyone at ease.

Is this what is called chaos engineering?

That makes a lot of logical sense. Every single ms counts in those cases.

It reeks of code base problems or general misunderstanding of systems administration. Easier to just make sure you're not leaking memory (profilers exist) and actually monitoring the servers than having to reboot your servers just in case.

Things like heap fragmentation still happen.

Now I understand why Jane Street uses OCaml. It has less of the GC problems.