As I mentioned, the issue isn't that.Yes, back when the National Dex was actually in the games.
It isn't anymore.
The issue is that with entire pokedex allowed in 2018 VGC (minus mythicals/covers), it means you *need* to be able to obtain legal legendaryes in the generation. Every other Pokemon you can just catch in another and breed down, expecially if as expected Tutors will reappear as usual in USUM, but Legendaryes you can only obtain in other games right now.
If they meant to exclude all legendaryes, the post with the rules would have said "excluding non alolan legendaryes", it only says "excluding some legendaryes" however, which highly hints at the presence of all others catchable.