Semoochie:
It really isn't...except for the technicality of saying "KFMB" all at once.
There's some history here.
The legend goes that when Bobby Rich took over as PD of KFMB-FM, then an automated Beautiful Music station. to take it Top 40 in 1975, he wanted to change the call letters, but was told no.
He then had a conversation with Charlie Van Dyke, who was doing the imaging for KFMB-AM, about the challenge of making the legal ID sound different from just "KFMB-FM". Van Dyke suggested that the ID, which would be done live by the jocks, have the call letters read as "KFM......BFM", with the emphasis on the "K" and the "B"...so:
"KAY-eff-emm....BEE-eff-emm, San Diego! It's (time) with (jock) as B-100 boogies on!"
If you want to hear what the inspired madness of B-100 sounded like, here's a link to a composite of its first anniversary in 1976:
https://youtu.be/KFVNHbI6FO0?t=27
Well, people still remembered and talked about that---and so, when "Jack" finished its 13-year run on 100.7 three years ago (after about 15 years as "Star 100.7), they decided to try branding the station as KFM-BFM. Here's a link to audio of the format change, which contains the logo artwork on the page:
https://formatchange.com/jack-becomes-100-7-kfm-bfm/
But too much time had gone by---it really didn't mean anything to the target demo. So now they're "100.7 San Diego".
It's all technicalities, but the last time that 100.7 used KFMB-FM as anything other than a legal ID without doing something cute to the call letters in terms of phrasing, was March of 1975. As David rightly points out, the call letters have no real value for the station.
I think the AM is trickier---since it has never had a station name--it has always been identified by call letters and dial position---for 78 years. And it benefits from the association with KFMB-TV. But we don't know what Local Media plans to do with the frequency, so it may not matter at all.
But, for history geeks, longtime Southern Californians and the like, it's a set of calls that began on radio that won't exist on radio anymore. It's worth noting, but maybe not mourning.