I don't know what the tipping point is, but it'd probably take more than one. I've commuted through backed up/stopped/solid red sections on a motorcycle where I just get to lane-split and cruise through, and it'd still stay red.
Which makes it all the more infuriating that Google Maps still has huge flaws when it comes to motorbike navigation.
While they could, I doubt they do... do you have evidence of this?
I would guess (unsubstantiated) that it classifies a motorcycle trip as being unexpectedly fast through many traffic jams.