He was asking for the (nonexistant) on-screen quote where it is stated Chekov had just came aboard as a member of the crew.
There never was any.
Hence the part about there being "several non-canon explanations"? If it had been explicitly stated in dialogue such that it could be cited and quoted, what use would anyone have for a non-canon explanation? I suspect that Noname Given
did see the "recently transferred" line somewhere, but may be remembering something from a novelization rather than a televised episode.