The people who designedand wrote Link are entirely familiar with JACK (if only because I taught them about it).
I too was a bit disappointed when Link was announced (last Novemeber) because it seemed redundant given JACK transport. But once they released the SDK for iOS and later the code for all platforms, it became clear that the Link team has come up with something quite different, extremely useful and really rather clever. Even just their clear identification of different kinds of musical time sync is a huge contribution for those of us who think about such things.
Ableton is actually full of quite a lot of software developers who are into open source. I don't know why there needs to be the level of disdain and skepticism for the company itself just because, like most other s/w development companies, they use a proprietary model. Their documentation for their Push2 surface is an exemplary example of how any company (even an open source one like Monome) should and could document a hardware device and how to interact with it. Likewise, their release of the Link SDK as GPL code for all platforms is a remarkably strong statement from a company whose core products are all released under proprietary licenses.