The US was bombing Japan during the Korean War?
No, he's right. These were planes that couldn't return to their home base during raids on Japan during late WWII, and were forced to put down in then-neutral Soviet territory. Those three planes (exactly three, important later) were then used for testing and as the template for the Tu-4.
After production began, three "B-29s" flew over the May Day parade, where western observers assumed Stalin had the three ditched planes repaired and put in his own service... as they approached Red Square though, suddenly a fourth plane appeared and joined the formation. Wait... we lost three, so where did that one... oh. Oh no.
Soon the sky was full of Tu-4s flying over the crowd in small groups... sort of. The Soviet Air Force was landing them on a nearby empty stretch of highway, painting a new number on the tail, and sending the planes back over the crowd so it would look like they had hundreds of them in service (at this point there were only a couple dozen)
So yeah, grimlock1 is right in this case. By the time the Korean conflict rolled around, the B-29 was no secret to the Soviets anymore- any lost over North Korea were likely very interesting to Soviet 'observers', but there wasn't much to learn that the examples they already owned couldn't teach them.
(Now, the B-36... that was different. Likely as a result of the Tu-4 fiasco, my granddad was under strict orders to destroy any secret equipment- bomb sights, radios, etc.- in case of a crash. Just in case. His expert opinion was that they should have gifted a Peacemaker to the Russians so they'd have to suffer through using the god-awful things too.)