it's way beyond plausible. the proven willingness to sacrifice souls in ww2 basically proves it. the space race wasn't very long after the war. it was the same people running the show.
But, if it happened, the CIA must have known about it, the same way the knew about Komarov. So, why are American keeping quiet about it as well? Do we need to extend this conspiracy to the American side as well?
and blow the cover of the agents who got you the info?
Not then of course, now, after the documents from the era were declassified, and the agents in question are either dead or long retired.
Spy agencies are good but they're not omniscient. It's entirely possible that things happened that the CIA didn't know about, especially back before spy satellites were a common thing.