This isn't entirely true. If you can't feel the ribs at all, then yes, your cat is not only overweight, but very much so. This is a good test for checking definitively for overweight or obesity, but it shouldn't be used to dismiss concerns about weight. For instance, if you can feel the ribs just by gentle petting (like gliding your hand along), that's probably okay (but to be thorough, you also want to check other areas like the spine and the cat's figure, which here very unambiguously shows overweight or obesity). But if this area feels "like the palm of your hand" and you have to apply some pressure to feel them, they're likely overweight, and you should try weighing them on a scale if possible or just ask your vet about it.