So, in the Galaxy client, I see that Cyberpunk Ultimate edition, which contains both the base game and PL, is listed as a DLC. I've been meaning to get it when it goes on sale but I'm confused by it being listed as DLC. Can I purchase UE by itself or do I still need to purchase the base game?

The Ultimate Edition contains both Cybberpunk 2077 (the basegame) and Phantom Liberty (the expansion).
You can play play the base game on its own, obviously, but Phantom Liberty requires the base game to work.

So no, you cannot play Phantom Liberty on its own. And why the "Ultimate Edition" is listed as a DLC, I have no idea, because it contains NOTHING more than Cyberpunk 2077 and Phantom Liberty.