Hi Professor Crossley!
Yes, I think it's been argued before that maybe that perhaps Tungusic was a family by itself. I'm not sure though, cos' the linguistic borders are so porous and words just enter the lexicon across languages so easily. My gut is that Tungusic still belongs to the Altaic scheme, and the spread of words was from Turkic to Mongolic to Tungusic - so maybe Tungusic is the youngest of the three?
But like you pointed out during the discussion, the Tungusic peoples have been more populous, so it's strange why the spread didn't go the other way too.
That's something I've been thinking about since yesterday