My Flow rims had spoke holes and the profile looked pretty normal to me, so why can't you put Stan's yellow tape on any rim and make them airtight? This is probably a huge dumbass question which I will regret in a few minutes time…

You can use Stans yellow tape to make any rim airtight.

The main difference with a Stans rim is the way it locks the tyre bead in where as a standard rim doesn't.

I'm pretty sure that when Stan's kits first came out they just used the tape. The rubber rim strips are a more recent addition and help to prevent the tyre 'burping' air out when cornering hard.