Valve's popular handheld gaming PC recently passed 14,200 officially rated games, meaning there's never been a better time to own one.
This is just talking about games approved through Valve's verification process. There are a lot of games that work that are "unverified", not to mention the entire history of gaming available through emulation.
The steam deck library includes the entire switch library via emulation, so Yeah Obviously. (Ik they're not counting emulation, but my point is that the steam deck is a PC, which makes it much more versatile)
They just talk about Steam games that were verified by Valve's process. So yeah, as you said, it's much larger for people who are willing to do a tiny bit of tinkering 👍