No they couldn't, it's fucking Linux. They'd have to tie the controller drivers hostage to "lock it down", and at that point they'd hit so many hiccups with legitimate users.
Like they'd have to pull so many things from Linux (in particular Proton) to "DRM-ify" the steamdeck.
And as I think someone else just posted, some of the stuff they'd need to lock-down aren't even things Valve has control over. Like I said Proton but Valve doesn't own proton.