That's a valid concern, hopefully Lenovo is smart enough to not insert themselves in the software pipeline between Valve and their customers.

The way it works with handhelds right now (Bazzite instead of vanilla steam OS), there's certain things that have to be enabled to get the hardware side of things working (all the buttons and inputs etc), but it's still really easy to both update and to use and I haven't had any problems with the ROG Ally X or the Lenovo Legion Go running Bazzite. I dual boot windows (and have group policy edited pretty much all of Microsoft's BS tracking and AI to banish it to that shadow realm) for windows only games that I couldn't get to run reliably in proton, but I'm really happy with it and I'd imagine steam OS coming to such handhelds officially would benefit users rather than being a detriment as far as ease of use goes.

A big problem with Android is ARM vendors don't upstream anything in so you need to run very specific kernel and bootloaders just to boot the OS. SteamOS on x86 won't have that problem, regardless of who makes the end device.