That's an interesting cause I'd never heard of. That may be the reason SA lines start to sink... or at least one of the causes... After all, Rio lines with their mono core don't sink, so there's a logic there. Of course, if you look at Cortland, Teeny or Wulff lines (all of which have braided cores) you'll find they keep floating, so that sort of falls apart.
Also, they way I attach my leaders to flyline is by loop-to-loop. I make my own loops in both ends of the flyline by doubling the line back on itself and tying a pair of Nail knots with 10-12 lb test mono about 1/2 inch apart. These cinch down so tight that I seriously doubt water is getting in through the ends. Personally, I think it has to do with the bouancy of the line itself, and maybe the diameter. (Greater diameter equals greater displacement equals greater bouancy... maybe.)
I haven't fished SA's new line, and I won't until one of my clients brings one with. All I know is my experience with their products while wading has been disappointing, to put it mildly.