'No doubt' is a figure of speech. But in species where there aren't any pain signals at all, my case is strengthened. I've skimmed the page you link: doesn't it just assume that 'pain experience' implies suffering? But this is contradicted by the testimony of cancer patients on opiates or Hindu saints undergoing surgery without anaesthesia. They feel the pain but it doesn't bother them.
Why are you so certain? There seems to be plenty of debate on the issue: https://en.wikipedia.org/wiki/Pain_in_invertebrates .