The M stands for Medium, Vibram makes Soft, Medium, and Firm, if a disc is only referred to as X-Link, that means Medium (as compared to X-Link Soft or X-Link Firm).
I am a huge fan of Vibram putters (especially the Ridge, and I am testing the Sole these days), but I think soft and medium plastics from any disc manufacturer have really limited applications.
You may want a soft or medium disc to lay down and stick on short approaches, but they do not perform well in most putting and throwing situations and they generally fail when thrown hard.
When you putt or throw a disc, you do not want the disc to buckle and then have to correct itself before obtaining its flight plane and appropriate rotation, you want your touch, timing, and energy to transfer directly into the disc and the disc to respond commensurate with your effort.
If you watch the Ibex demo video that Steven Dodge posted on this thread (it is also on the Vibram Disc web site), you can actually see what I am talking about since the throws are in slow mo, you can see the Ibex buckle and wobble, which is an undesirable outcome for most throws and putts.
I am not sure why Vibram only released the Sole in Medium, and I am worried the Ibex will only initially be released in Medium. I can not wait to try the Ibex, but will likely wait until they release it in Firm, because that is the only way to really assess the overall performance of the design and plastic.
