You betcha, Jim! Your making assumptions. I could do that too. But it's not proving or disproving anything. How do we know that our green hazy beer does not have enough suspended solids, regardless if it's yeast, protein, starch, hop polyphenols, to effect our hydrometer reading? What instrument or calculus are we using to determine that?
My hypothesis would be: 1) of course anything but water in water makes it denser 2) any instrument that measures density will, with proper use, reflect that equally well (no preferred instrument) and 3) at the point you are taking FG readings there will be so little suspended yeast left, that it will be an insignificant contributor to the density of the sample, far smaller than the margin of error in just reading the instrument (unless that instrument costs as much as your house and is in a university lab.)
A method that might throw light on this is to take a FG sample and measure it, then refrigerate it for a few days to settle any yeast and read again. The problem I see with this is the possibility of continued fermentation. I may not be able to contribute much there, because a) I take my sample after crash cooling anyway and b) I know my yeast (34/70) will in fact keep fermenting after crashing.
But hopefully you'll get enough citizen brewing scientists going here to get a real answer! As OP you are the official proctor. Good luck.
I said it's a hypothesis, that's my point, it needs to be tested. Somehow we need to see what yeast all by itself does, because we expect all that other stuff to be in there too. (And in the end, if you know that x APPARENT attenuation means it's time to rack, does it matter what all the floaties are, or the REAL attenuation?) Still this will be good to know. Everything's good to know! Looking forward to it.