There's no doubt that LLMs are "dumb" in many aspects, but without exploring why they fail that doesn't tell us whether this is some inherent lack of ability to represent reasoning or understanding, or simply holes in their training. There's no reason to assume the training data LLMs currently learn on is in any way equivalent to the data a human child is exposed to growing up, so there is no reason to assume it will have the same strengths and weaknesses in what it is able to reason about and that we can therefore draw conclusions about it's overall ability just from probing something that appears like it should be simple to a human.
(and incidentally, I think people here will overestimate massively how well humans would do on the consonant test above; even native English speakers without any recent exposure to being taught rules as opposed to "just" using the language - many would be able to do it, but I'd be able to bet many would struggle, though most who'd struggle would express that doubt)