He quoted the "domain expert" part, I'm pretty sure he didn't mean real domain experts, rather just the local programmer who happened to take most of the tickets related to that domain.
For example, early in my career I worked at a small company. They had an "SQL expert", but I got better at SQL than him almost immediately since he wasn't really an SQL expert. I wouldn't call myself an SQL expert today either, its just that the "expert" didn't even know the basics or how to work with indexes properly, what types of queries are fast etc. So he wasn't an SQL expert, he was just the guy who did tickets related to SQL, and then managers starts calling him an "SQL expert" to make his team sound more competent.
But if we take a real domain expert, for example a Math PHD, then not a single genius ever could learn enough to catch up within a weekend or two. Every genius in history in these domains has spent years learning them. I don't think anyone would claim to be able to do that in a weekend. But get better at SQL than a typical backend developer over a week? Yeah, I'd believe that.
EDIT: Saw that the above comment mentioned PHD. Yeah, then it is a bit ridiculous, at least if he meant that the person learned the equivalent of an EE PHD over a weekend, that wont happen. Maybe the guy found some error in the works of the expert and people blew that out of proportion.
I'm leaving the rest of the comment as is, but I agree with you, his story is a bit ridiculous if you take it at face value.