Look, I love this game as much as the next person but is Hacker News really the place for deals on games?

If you think it doesn't fit (and you have enough karma), flag it.

I registered just to get a freebie, and got 3 separate captchas, with the same "click on the vertical river" selection with seemingly same options... why? why??

To prevent bots from claiming them and then selling them on shady 3rd party sites

I've definitely played this game on a Mac. The Steam store does sell it: https://store.steampowered.com/app/255710/Cities_Skylines/

Games are to be written, packaged and sold. Not continuously maintained to keep system compatibility. Apple philosophy of dragging developers into the future clawing and screaming is fundamentally incompatible with games. (64 bit transition, OpenGL deprecation, full screen mode changes, arm macs…)