Back in the day? It seems like git just came out, is just a few years already like ancient times? I don't get why they can't use git? Git is used to maintain the entire Linux kernel and they want some system rarely used?
I wish people could make up their minds on what DVCS we should all be using and stick with it for 20 or 30 years.
They are presumably trying to use a single repo for a very large chunk of code. Large, not in the sense that the Linux kernel is large, but a repo one (maybe two or more) magnitude larger. Multiple gigabytes large, with most of those gigabytes taken up by small objects.
Git does indeed start to fall down with these sorts of repos. The thing is, if you keep on growing like that, other VCSs will as well. Even with perforce you'll hit a wall eventually.
The solution is to restructure your project, breaking it into several different repos. Understandably, large organizations capable of creating large repos like this are typically going to be resistant to large changes like this brought on by what they interpret as limitations of tooling. Doing it requires restructuring code, training, and may require extensive modification to internal systems that work with your codebase.
My perspective is that this 'limitation' of git is really a symptom of an anti-pattern that should be corrected sooner, rather than later, before you find yourself in a really painful situation (read: three years later your repo is now 10-100 times as large, and you are discovering that continuing to scale with Perforce is becoming untenable).
I know this is a bit meta, but this seems to go against the entire spirit of git.