They could have easily crammed the Steam Deck full of stuff to make it hard to use for piracy - locking down everything, making it usable only to play games you legitimately own, force you to go through who knows what hoops in order to play games on it. That's what Nintendo or Apple or most other companies do.
But they didn't, because they realized they didn't have to. It's 100% possible to put pirated games on the Steam Deck - in fact, it's as easy as it could reasonably be. You copy it over, you wire it up to Steam, if it's a non-Linux game you set it up with Proton or whatever else you want to use to run it, bam. You can now run it in Steam just as easily as a normal Steam game (usually.) If you want something similar to cloud saves you can even set up SyncThing for that.
But all of that is a lot of work, and after all that you still don't have automatic updates, and some games won't run this way for one reason or another even though they'll run if you own them (usually, I assume, because of Steam Deck specific tweaks or install stuff that are only used when you're running them on the Deck via the normal method.) Some of this you can work around but it's even more hoops.
Whereas if you own a game it's just push a button and play. They made legitimately owning a game more convenient than piracy, and they did it without relying on DRM or anything that restricts or annoys legitimate users at all - even if a game has a DRM-free GOG version, owning it on Steam will still make it easier to play on the Steam Deck.
Steam itself has some kind of DRM. You need to login to Steam to access the games you bought (sure there's offline mode but then you can't download your games, update or buy more, so it's only temporary convenience). If Steam dies one day, so will your Steam games library.
However, the service is great, so it's not annoying.
Yeah. What I mean is that the Steam Deck itself doesn't add anything special in that regard to fight piracy.
(Plus, I mean, Steam's base DRM is like a screen door or a "please do not pirate" sign, lol. If Steam dies one day, Steam DRM won't be a problem because you can basically crack it by breathing on it too hard. I assume that is purpose is to ensure that you have to violate the DMCA's anti-circumvention provisions to pirate their games, not to actually slow down pirates at this point.)