IF Krist's work is the earliest such discussion, in fiction or non-fiction, AND there is no acknowledgment of Krist, morally I'd say that's plagiarism. Plagiarism per se isn't a crime technically. If the work were still under copyright at the time Aldiss published, and that seems probable, I SUSPECT it would be copyright theft, which is both a crime and a tort, but since it isn't identical, that would require expert opinion, and mine is not.
Having said that, given that both writers are prominent, it is hard to believe that those "ifs" above apply. Hard to believe Aldiss would be that stupid. But you never know - stranger things have happened.