Sure, Loki proved himself to be sufficiently strong to Ragnir, but what would happen if Ragnir chose to keep fighting him? Ragnir got a bump on his head from the clash, but Loki had his hammer completely shattered. Wouldn’t Ragnir beat an unarmed Loki?
This isn't a case where Ragnir decides they don't need to fight anymore or whatever, he becomes Loki's follower for life. Relativity isn't really a thing here
