I just assumed that Valve ran out of the APUs AMD made for Steam Deck LCD.
There were news/rumours that it was originally designed for Magic Leap 2 and Valve got the leftovers for cheap: https://www.tomshardware.com/pc-components/cpus/steam-decks-... .
If they're going to spend a premium on ordering a new batch, they might as well order the APU for the OLED model they charge full price right?