Sorry, you do have to buy it again on itch.io to download it. I also bought it on steam and learned the hard way that you can't even install windows programs on mac, even if they let you buy it. On the bright side, its fairly easy to get a refund from steam, just fill out the refund form and tell them that the game doesn't work on your OS. I got the refund from steam within 3 days and used it to but Termina on here.

Step by step instructions for playing Termina on Mac

1. Go to playonmac.com/en/download.html and download the appropriate version for your Mac (4.4.3 for MacOS 10.15 and above and 4.3.3 for MacOS before 10.15
2. Move it to your applications folder
3. Download Termina off of Itch.io, NOT Steam. Steam will not let you install it on Mac.
4. Click on your Termina zip file and expand it so you can see Game.exe
5. Open PlayOnMac
6. Click on “install a program” on the left hand side
7. Click on “install a non listed program” at the bottom of the screen
8. Hit next on the next three windows that pop up
9. When it asks “what would you like to do”, choose install a program in a new virtual drive and hit next
10. Type a name for your applications virtual drive
11. Don’t click anything on the next screen, just hit next
12. Choose 64 bits Windows installation on the next screen
13. Wait until it’s done loading, and then browse your computer for your Termina game.exe file
14. Click open
15. Hit next
16. Termina should open and run just fine!!
17. Hit next
18. Choose a file for PlayOnMac to make a shortcut (wmplayer.exe)
19. Choose a shortcut name for wmplayer.exe
20. Play the game!