Spanking is legal in all 50 states as long as it is not considered to be excessive.
Also, unless it is specifically stated in a court order that Clevinger is not to spank his child, your point about him not having primary custody would not matter and it would still be legal for him to spank his child.
Assuming there is no court order with this language, none of this would constitute child abuse in the eyes of the law, which is why I thought it was strange that the video in the tweet was even shared by whomever the “Goblin Gobbler” is. My guess is that this is also why the White Sox did nothing with this video, as it proves nothing.
Finally, no one mentioned using a belt until you did. That may be considered “excessive” which could be deemed child abuse by the courts, rather than legal open-hand spanking.