I'd actually dispute that. The tech world is really, really big. There are a great many extremely talented engineers that have never worked at FB or Google, but they're also the kind of engineers that don't write blog posts or attend conferences, so they're less visible.
If she'd stayed in Chicago (Northwestern grad), that's unlikely. Of course those companies have outposts there, but fewer tech roles by an order of magnitude or two.
I know quite a few of them (and would like to believe I'm talented, too).
What percentage of developers that is I haven't the faintest clue. But definitely significantly more than 10% of my friends have worked for at least one of FAAMNG. And I have a lot of non-nerd friends too.
The propinquity is insane. When my GF decided to leave MS she walked from her MS building on her lunch break to her interviews at Google and LinkedIn (LI has subsequently moved). Of course the inverse is true too: there aren't many startups in that area, except in the Landings complex (where Cygnus used to be!).
citation needed