That's a tricky way to look at drugs that end up on the banned substance list. The article says that this drug isn't even on the list right now so I'm not sure it's something they would even get in trouble for today.
But, say tomorrow some new PED comes out and it works wonders, maybe even better than stuff like steroids or HGH. Obviously it will take a little time for the NFL to catch on to it so for all the players who use it until it gets added to the list, what do we think of them?
It wasn't banned at the time they used it. Of course, it wasn't banned because nobody knew it existed.
Doesn't change the fact that the players taking it were trying to get an advantage they wouldn't be able to get, or would have to work for years to get, without the drug.