tournaments rules: USCF

It was a rule violation but I seriously doubt if the TD would have done more than warn the player of the requirement to move first and then record the move.
A lengthy and intelligent discussion of this issue-
https://www.chess.com/forum/view/general/uscf-rule-15a

Writing first and then moving is allowed under the rules for USCF, though the reverse is preferred.
The move made on the board is the move that is counted, so if the move made on the board differs from the one written down, the move on the board is all that matters. If the player was writing a move down, erasing it, then writing another, that could fall under note taking, if done very often, but would garner a warning first, at most.
As far as So's situation, that was a FIDE event, he had been warned, and he was making notes of a different sort. That was slightly different.