A data structure cannot be functional. I understand what he's trying to say, and agree with most of it, but the word "functional" is purely wrong. What he wants to say is "good". But not all "functional programming" is good, nor is all good programming functional by necessity, despite what your local Lambda The Ultimate nerd tries to tell you.

The Best, when it comes to data structures is a Directed, Acyclic Graph. For instance your typical linux filesystem is a DAG. But there's one problem with DAGs: When they reach a certain complexity human brains are not fit enough to parse them anymore. (programs still can though)

So in many circumstances at least a human programmer needs to take a look at the state of your program and make assumptions about its correctness, which is called debugging. And that's why in Good programs we often use Good data structures instead of The Best.

Good data structures are key->value stores (which you may know as "hash tables" or "dictionaries"), trees, and trees in a simplified special form: lists, each of them being somewhat able to represent the other two, if one can accept a performance hit and/or increased complexity in source code. Dictionaries, trees, lists. That's it. And you do that in every programming language that is at least a little bit interested in being Good.

So there's nothing special or functional about git's data structures, it's just normal Good programming, and a few programmers who are so good at programming that they don't even need to mention it anymore, they breath good programms.

Then of course to the normal bread-earning coder good programs are a rare sight. But the reason is not that they are really rare, the reason is that successful business doesn't really require Good programs to succeed. Mediocre programs are good enough to earn their rent, and most of us spend most of our coding hours to earn our rent.

All that being said, if you don't just want to make money, go and spend some time studying git internals. It will teach you a lot more than most of your teachers/professors taught you combined. Sadly the source code is written by Linux gurus, who like to encrypt their source code with a very special key that only people from their tribe can understand. But the Git Book is actually good enough that you can study quite a lot of the internals from that book. I also suggest writing your own git in your favorite programming language once, to really understand it.