True, but there are items that support even hybrid pet builds, even though currently there is no real way to make them work.
And again, it is not the Pet’s lack of defensive options. It is the player’s. Pets have no problem surviving even in setups that do not go for Ishtak or Tree of Life. For eg: all the Mogdrogen based Pet builds. However, the player’s survivability would become an issue.
To give an example: Sanguine Lupus - Pet Conjurer does all the superbosses easily, even Callagadra. But take it to high SR and you will die a lot due it lacking healing for the Player.
Pets die against Cally (using it as a reference) in the above build just as much as in my The Fluffy Squishy - Pet Conjurer even though this one is mch more defensive.