I think when people call a class squishy nowadays, they’re really referring to the general idea of the class, not so much anything as specific as their AC or their hit dice. Rogues (for example) are “squishy” not because they can survive meaningfully fewer attacks than fighters (though that is technically true), but because they’re the teammate you don’t want to put on the front lines. They’re at their most effective when they’re skulking around, taking opportunistic strikes, and are at a disadvantage when forced to be in the thick of things. I don’t think a lot of people are really running the math, they’re just going by general feel, which is heavily informed by biases and preconceptions. The squishy rogue is a trope, so it’s what people expect out of the rogue, so it’s what they get out of the rogue.