Apple is good at deflecting blame so there is no sympathy for Apple here. Even if it is a mistake by Apple, I am confident it will push it on to the alt store and the fans will lap it up."Apple will get blamed for third-party App Store hack" doesn't reinforce your argument that third party app stores will have to be better than Apple; it refutes it.
Also, why are you assuming all third party app stores are going to be rational, safe actors? You don't think China or Russia might have reason to spin up a third party App Store and trick users into downloading apps that compromise security and/or privacy?
All I am saying is that it is not for Apple to worry about the alt store app security. The alt stores could be better at security than Apple as Apple is not setting a high bar currently. If the state actors want to compromise app stores, there is nothing apple or anybody can do. Do you think Apple will vacillate and hedge so much if China were to bring a similar law? Apple will go through hoops for China.