The 'X' does indeed indicate that the cross is non-fertile. This is why it's not a species: members of a species can breed to produce "vigorous fertile offspring", which is not what we have here. Reproduction by runners isn't "breeding true", because it's not breeding. It's asexual reproduction. Reproduction from seeds isn't "breeding true" unless the seeds produce the same kind of fruits and plants as the parents. If you have evidence to the contrary, please would you provide us with a suitable link. Thanks.

Loganberries do not form a separate species, but are a hybrid. If you believe otherwise, please provide an unambiguous reference from a respectable source that supports your claim. Thanks.