...By Thasians, who were pretty impressively alien themselves. We learned next to nothing about Antosians, but supposedly they were quite vulnerable to Garth's conventionally armed rage, and thus probably not in quite the same category as Thasians.
Or then Garth posed no threat to the Antosians, but he and his crew were fooled by these superior beings into thinking that the threat existed, just like the Organians fooled Kirk and Kor.
In any case, even though Garth learned "cellular metamorphosis", which sounds awfully physical, and used this to "restore" destroyed body parts, which again sounds like actual physical altering of one's dimensions, the effect we ultimately see does not
appear to be one of body part modification. When Garth changes appearance, things like clothes change as well. Is Garth actually naked and exposing the outer layer of his favorite or "default" body when we see him dressed up in his silly poncho and boots? He retains this look when knocked unconscious...