I don't think macOS Safari and iOS Safari are the exact same, I may be wrong but that's what I think. I remember a few years ago the AppleTV couldn't play 4k YouTube videos and that was because Google used a format that Apple was against implementing(Because it was inferior to what Apple already had if I remember correctly)

Pretty sure it's because Safari doesn't support the codec they use, AppleTV had this issue for a long time aswell