That is probably true. If there are six bombs being smuggled separately and one is intercepted, the country would turn itself inside out to find any more. The effort would be extreme, to say the least.
Zahawri originally wanted to hijack 10 planes, crash 9 of them and then land the 10th and read a message to the media. Bin Laden thought it was ridiculous and would be foiled. He went with 4 planes and no airport press conference.
I am sure they would prefer 5-6, but the risk is too high.
I also think the idea of 1950s and 1960s era USSR nukes being planted is just an urban myth.