If he were a “Bush lackey”, Dubya would’ve supported him or endorsed him (or at least had kind words for him). Dubya went out of his way to attack Cruz personally, not exactly a demonstration of his being a toady. Dubya knows Cruz is serious about undoing the damage he inflicted by not enforcing immigration laws, and that will earn him his eternal enmity.
I do have to wonder what the conversation between Dubya and Neil is like these days, given the former’s disgust for Cruz.
I think Bush’s harsh words for Cruz were meant for public consumption and don’t necessarily represent his true opinion of Cruz. It’s also possible the Bush doesn’t like Cruz personally but nevertheless views him as an ally.