But presumably the attack wouldn't be replacing the screen -- which is what this workaround allows -- it'd be replacing the chip with one that you've compromise

But presumably the attack wouldn't be replacing the screen -- which is what this workaround allows -- it'd be replacing the chip with one that you've compromised, because then you could have it let you bypass security. Not to say that they won't find a way to get around that, just that the existing workaround doesn't create a security issue.