The actual accusation -- if you read further down -- is that not only was Oppenheimer a spy, but so were Fermi and Szilard and Bohr. The evidence for this comes from a Russian ex-spy who was imprisoned in the Soviet Union and tried to cash in during the 90s on these accusations. You can get the fact check if you dig around enough, I won't bother here.

Based on some Googling, it seems these unfounded accusations have been a popular theme for some decades. Makes me wonder what all four of those scientists have in common, that would inspire such reactions.