Okay, for starters, I have to say that I have not installed these specific windows
(although I will be re-doing mine this spring).
That said, I have installed hardware that utilizes gaskets, and I don't combine a gasket and a sealant. Gasket or sealant, but not both. I mean in the overall sense, because I could potentially see using a dab of sealant on the corner "seams" of the backframe gasket (but not slathered on underneath, like I think is being recommended). I would have see how the corners fit.
Also note that I'm not dis-recommending using butyl along the top edge, because the backframe gasket doesn't go there (only goes on three sides).