That kind of depends on how you define FOSS. The way we think of that today was in very early stages back in the 1991 and the orignal source was distributed as free, both as in speech and as in beer, but commercial use was prohibited, so it doesn’t strictly speaking qualify as FOSS (like we understand it today). About a year later Linux was released under GPL and the rest is history.
Public domain code, academic world with any source code and things like that predate both Linux and GNU by a few decades and even the Free Software Foundation came 5-6 years before Linux, but the Linux itself has been pretty much as free as it is today from the start. GPL, GNU, FSF and all the things Stallman created or was a part of (regardless of his conflicting personality) just created a set of rules on how to play this game, pretty much before any game or rules for it existed.
Minix was a commercial thing from the start, Linux wasn’t, and things just refined on the way. You are of course correct that the first release of Linux wasn’t strictly speaking FOSS, but the whole ‘FOSS’ mentality and rules for it wasn’t really a thing either back then.
There’s of course adacemic debate to have for days on which came first and what rules whoever did obey and what release counts as FOSS or not, but for all intents and purposes, Linux was free software from the start and the competition was not.