The BIOS doesn't usually handle encryption, that's usually the bootloader on the drive itself. They could just reformat the drive with the standard Valve image and they're good to go.

It could protect your Steam account though, so if you're worried about them making illegal purchases or something, it can help somewhat. But most thieves aren't that sophisticated, they just want to resell it for quick cash.

In order for the BIOS to work, you'd need to have some kind of cryptographic link with the boot media, something that the standard Valve image wouldn't satisfy. But let's say you do that for your own device, all that does is annoy the thief, it's not going to prevent the thief from stealing your device. Now if every Steam Deck did that by default, maybe thieves would be less interested in stealing it, IDK (probably not, I doubt Steam Decks are popular enough for thieves to now how stealable they are).

I personally don't see the point. Steam Decks typically don't have sensitive, personal data on them that needs to be wiped, so bricking them doesn't benefit the original purchaser being a small amount of "justice" at knowing the thief just stole ewaste. I'd rather a thief resell it and someone get to use it than it just be tossed in the trash.