Why did the Soviets use FM to jam AM radio broadcasts in Estonia?

[bitrex]

I found the following extremely interesting article about the jamming of pro-Western broadcasts in Estonia during the Soviet period: http://www.okupatsioon.ee/english/mailbox/radio/radio.html"

One part of the author's description I'm puzzled about: "Although all LW, MW, and SW broadcasts were always amplitude modulated (AM), amplitude modulation was not used for jamming, but rather frequency modulation (FM). And this, on such a narrow strip as is needed in a band for 1 station, that is, 9-10kHz. If the transmission bearing wave is viewed with an oscillograph, the modulation cannot even be observed, as if it didn’t exist."

I'm not sure I understand why FM was used to jam AM signals - and what the last sentence means exactly. Which transmission can the modulation not be observed on, the jamming signal or the incoming broadcast?

 

[waht]

A jammer sends a stronger signal to overpower the weaker receiving signal. To be more effective, you have to cover the entire bandwidth of the signal to be jammed. That would require many fixed frequency transmitters. But instead, one jammer can be used with varying frequency.