Strategy sessions with your supporters is not stabbing anyone in the back. I am sure that Trump has strategy sessions with his supporters too. An attack is made publicly to try and damage your opponent. Cruz’s statement was not made publicly, instead it was made privately to his own supporters so it was not intended to damage anyone.
When your private session with strategists include dissing Trump for his judgement and putting it out there about finger on the button, it’s a prelude to where Cruz will be.
Just doing it in private first so he’s not seen as poking the bear.