> Two of the largest tech companies in the world, Google and Meta, had to roll custom VCS for their day to day engineering operations because git and git submodule were so unsuitable.

Where did you get that from? Sources?

Google rolled their own VCS, because Google is older than Git, and they needed something that works. Their custom VCS is a hacked up version of Perforce.

By the time Git came around, Google was already pretty much committed to their in-house custom tool, too many things relied on it.