Ehhh, to be fair, I've seen plenty of laptops with the WiFi chip soldered in.
I agree, it's better to be able to replace it, but I get that it's a part that is more rarely replaced.
Also, all the internal testing Valve did was with WiFi 5. There's a small chance WiFi 6 could cause more interference and make the Steam Deck act wonky.
It's the state of affairs for modern laptops to have everything soldered, but the Steam Deck is big on upgradeability.
WiFi 6E might've caused a problem, but as far as I know, WiFi 6 works on exactly the same frequencies and powers as 5 and 4.
Also, I've run into a surprising number of thin-and-light ultrabooks with soldered RAM but standard PCIe WiFi cards. Ironically, one of the more upgradeable laptops I've had (from Clevo) is Intel CNVio v1-only, and guess what Intel conveniently doesn't make for CNVio v1.