This, mostly. Without going into the exact details of the size of the team (the number stated here isn't all that far off for Content Design), this is the biggest reason why mods are able to add much more flavor than the core game team is able to. Another reason is that, on top of having less manpower working on the lore in the first place, the implementation progress is also slower for the core team. We go through a long iteration process, and at every stage of implementation (and even before implementation is begun - research, design, draft, initial stage implementation, second stage (feedback) implementation, and further polish) the current state of a tree is scrutinized and receives feedback from at least half a dozen, and oftentimes multiple dozen, people. This takes extra time.
The extra hands probably help the mod project immensely. I don't play mods often, but if Kaiserreich really can add focuses without doing major bugs like this example in vanilla I can see why it'd be attractive:
This is far from the only focus related bug. It's not even the only serious issue with focus trees in China. This is also current patch, not WTT release state. When you make years-long plans in the game only to discover that said plans are tanked because the game lies, it's pretty frustrating. Sadly, this is not an isolated incident in HOI 4 right now.