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.
The steam deck is how you prevent piracy. If you look at the huge influx of streaming services, you'll see an example of how you encourage piracy. I recently dropped three of my services in favor of one pirate site that has almost everything. They even offer a subscription tier and I've considered it. I'm willing to pay for good content. What I'm not willing to do is pay dozens of middlemen across multiple companies to rip off the people who actually make my favorite shows and then memory hole the shows a few months after they premiere.
It's not the amount of services that's the problem, the competition is aleays a good thing. It's exclusives that are the problem. Almost noone is complaining about origin and uplay even thouhh it's games are available via a launcher launching yet another launcher. But epic? Everyone hates epic precisely because of their exclusive deals taking content off of other platforms. And for streaming, I guess if some of the players worked out some deal to get their hands on exclusives from other platforms, people would stop complaining about it, even if they jack up the prices to ultimately end up with the same amount of revenue.