DRM-free does not necessarily mean there could not be an option to sign in with your game when you want to do the challenges. Things like this existed befotre Steam or the Steam Workshop.
Unless there's something that restricts players from making as many free accounts as they want, to get as many trial runs as they need to learn the day's levels for an unfair advantage, this particular game mode is not going to work as intended. The point is to see who does best in a single run, not who is able to top the high score list by spending repeated runs memorizing how everything is laid out. Technically, someone could buy additional copies of the game for a similar effect, but the additional cost involved should help deter that. Having accounts tied to a purchase can also potentially allow for better moderation of the score boards. The entire thing making the Daily Challenge mode different from the normal game is the score ranking, and if that's open to easy manipulation, you'd might as well just be playing the regular game mode, which is otherwise pretty much identical.
More and more games seem to be tying features exclusively to Steam either because of the ease of piggybacking on the account, or because the Workshop is an existing framework for tools like level editors. This may be good for Steam customers, but it does not seem to be good for anyone who wants to be able to choose where to buy without being cut off from what could be called core features of a game.
If Steam makes it easy for developers to make use of an existing secure login system, that can help relieve a burden from small developers that often have a very limited budget to work with. And I'd hardly say that an online high score list is a core feature of the game, since the original never had one. Ultimately though, you do have a choice of where to buy the game. If a game includes extra functionality on a platform like Steam, you can chose to buy it on that platform. Or buy it at any other online store that includes a Steam Key with your purchase. GOG doesn't do that, so if you consider that functionality important to you, perhaps that platform isn't the one you should chose for the game. On the other hand, GOG gets an extra feature lacking in many games released on Steam, and that's having the game be free of any sort of copy-protection. Someone on Steam could likewise say that they're missing out on that feature. It all comes down to which feature-set you prefer.