Update: It still shows up in my Mac library. It said it installed, but it really didn't. Good old Steam must be glitched on my account. Tried starting it but it says it can't find the executable.
Deleted it and installed again. Noticed it doesn't even take 2 seconds before it says it's done installing. I didn't notice the first time because I had clicked on install and immediately walked away from the iMac for a while. Oh, well. Commence waiting.