"Music City Miracle" - legality of a forward pass

The “Music City Miracle” was a playoff game between the Bills and Titans, and the controversy was whether or not there had been an illegal forward pass on the game-winning kickoff return.
Regardless of whether that play was legit or not, what *would *be a legal forward pass in a kickoff play? The line of scrimmage was the kickoff line, so where could the ball legally have been thrown forward, by any Bills or Titans player?

By a Bills player, from behind the Bills’ kickoff line, towards the Titans’ end zone?

By a Titans player towards the Titans’ end zone? (But that would be a lateral.)
Or is this simply a play where no legal forward pass is possible?

No forward passes are allowed after a change of possession, including kicks.