Different take:
A solid accusation of being a commie is just the cover that Lem needed to stay out of trouble back home in Poland.
That might have been a more interesting hypothesis about motivation if Philip K Dick was denouncing Lem as a communist propaganda on stage at a convention rather than in a private letter to the FBI
Ah, but with all the moles, perhaps PKD was thinking a private letter to the FBI was a public letter to the KGB