Who's jammed with Bon Jovi?

You're absolutely correct.

You're right, Lemmy wouldn't embarrase himself playing with Bon Jovi these days, he rather associates with musicians who still care about music like Dave Grohl and Mike Monroe.

To jam would one not have to be playing an instrument?

Singing isn't jamming.

If you want to get technical, I wouldn't call playing cover songs jamming at all, even though there's some improvising going on.

Jamming to me is a bunch of musicians playing music off the cuff. I think covers count.

So why isn't singing jamming?

Yeah, you're right of course, the general meaning of the word is as you described above. Jamming covers gives less freedom to the players opposed to just selecting a key and jamming whatever comes to mind, both melodically and rhythmically. Then they aren't tied to any song-structure or dynamic.

i'm just waiting for some pretentious numpty to come along and say "my instrument is my voice"

You're talking like Lemmy is a music-genius. For good sake, if you have written Ace of Spades you have ppretty much lost your chance of being taken seriously.

If I remember correctly, he also came up with the epic song title "Killed By Death"...

I definitely didn't say Lemmy was a musical genius I do think he's more passionate about music and rock'nroll than Bon Jovi have been in a long time. I haven't listened to much of his music, but I've seen the documentary Lemmy and watched quite a few interviews of him. He's the real deal. Jon Bon Jovi thinks Taylor Swift is the real deal. Go figure.