No, this is / was my bad, totally completely my fault. My apologies.
I made a rookie mistake not checking that link was universal, letting a localized link be posted. Link I gave earlier is for Windows Store UK. Computer location has to be set to UK for that to work (Settings > Time & Language > Region & Language). Changing computer location to USA it did not work anymore, giving the "Not available" note.
Now computer location set to USA I tested again, changing the link from Windows Store UK...
...to Windows Store US...
... it works. Although still showing the "Not available" note, it also shows a working Install button:
I will edit the note in tutorial tomorrow, too tired now. It has been a long day, it's half past 1 AM here.