Discarding to Hand Size + Multi-Turn Loops
The Teferi player eventually (i.e., before Slow Play applies) has to choose to lose this game.
This loop includes the Teferi player taking an action; they can choose to take a different action. The Nexus player is simply doing what the game rules instruct them to do - yes, they're choosing which card to discard, but it's not a game action they're taking, it's following the rules of the game.
It's an odd outcome, and if someone rules this as a draw, there's no one in Policyland that's going to yell at them, or even say they're “wrong” - but if it comes up, and you're paying attention now, you'll know what to do.
d:^D