Eternity Professor, Phoenicialux | Great Nature G-unit Loop Hole?

Eternity Professor, Phoenicialux

Stride (Release it when both you and your opponent have a grade 3 or greater vanguard!)-Stride step-[Choose one or more cards with the sum of their grades being 3 or greater from your hand, and discard it] Stride this face down card on your (VC).
[AUTO](VC):When this unit attacks a vanguard, choose up to two of your rear-guards, and you may have those units get [Power]+4000 until end of turn. If you do, at the end of that turn, retire those units.

The problem is if the current vanguard is Leopard or Tester Fox in Legion mode, would their ability still get activated?

So far, I'm still waiting for the right answer on this matter, whether the loop hole is Legal or not. However, if it is Legal, then GN can be a really great deck on Stride!