You're right, my bad. This is how you get into stance from REhe does the RE then he does the B THEN he enters the stance
the way he's doing in the bug he transitions instantly from RE to the stance
The transition from AG to Prep happens as quickly as you can input it. If you pressed forward on frame 1 of Arriere Gambit's back step, you'd go to Prep immediately. In this case cause he's back turned after Taki walks around the 2nd 4B+G is actually a 6B+G. So 6B+G during AG is Prep RE. Basically performed instant prep by dumb luck. It's all technically working correctly though. Instant Prep is something they've balanced his combo damage around, and RE will always reorient anyone back to facing their opponent.
So how can you deal with it? I suppose with your current plan of attack you could continue your walk slightly longer cause he can't delay the prep version of RE, and it's pretty linear if it hasn't absorbed any hits.