No, you're not the only one. For anyone who has watched invertebrates like crustaceans or cephalopods, I really think this would be very obvious, save that there was a preconception otherwise. But generally it seems strange to me that we still approach it as if there's a line for suffering.
There's a spectrum of more and more complex behavior, gradually moving through taxis, reflex, instinct, and more developed mental models, from sponges up through to the apes and whales, but without any sharp divisions. Why would anyone not expect response to injury to go through the same continuous spectrum?
Still kudos on this key point:
Denying that crabs feel pain because they don’t have the same biology is like denying they can see because they don’t have a visual cortex.
I've actually seen that basic argument here: in mammals a particular type of receptor is used in pain and another in reflex actions, and crustaceans use the second for everything, so none of it should count as real pain. As if the precise choice of chemical messenger truly dictates the ethological role, let alone our ethical approach to it.