As you said, his later years
I think my post didn't come across as intended.
I was merely pointing out (aka inquiring about) something that I couldn't help noticing as a former early music performer. The only actual problem I have with this is that it doesn't allow me to get an independent and objective idea of what the instruments sound like (and by that I mean their acoustic voice, not what comes out of a processing chain) and how they match up to the violin.
Of course I didn't mention the recording equipement, that's the inevitable anachronistic part required for any recording. For which I assume that the sound techs (and the musicians themselves) always aim to get the most representative rendition of the sound a live audience would hear. (That said, I'd have appreciated this even more if it had been recorded through a single binaural mike set-up.)
And I certainly wasn't implying anything about the musical performance itself. I'm not the biggest Grappelli fan (nor is this what I prefer to hear on violin) but I wouldn't have had trouble believing it was him playing.