As the saying goes: "Form follows function". These are whole body lifts. The stronger the whole body gets, the more muscle the whole body gets. The swings release HGH (human growth hormone) in the body, and the biceps work plenty on the bottom portion of the swing.
Sure my arms were bigger when I use to do more bodybuilding type training when I was young. But I don't do any isolation exercises for them other than the curl at the bottom of the goblet squat, and they're doing pretty good.
I do find chin-ups are the best bicep builder for me, but my elbows are too sensitive now that I'm older. So swings work great without the elbow pain.
Al