Well, this does seem to be a new bone of contention in the yin world, so I will share my two cents worth of opinion.
The Sinew channels are superficial, broad channels that follow the muscles, tendons and ligaments (fascia!) They are very similar to Thom Myers myofascial meridians. They are conduits for wei chi (which strengthens the immune system). A few Yin Yoga teachers with a TCM background say that Yin Yoga does NOT affect the acupuncture meridians, but it has similar benefits to Qi Kong and Tai Chi, because It affects the sinew channels, not the acupuncture meridians. I agree with the 2nd point, but not with the 1st point.
It is possible to target the sinew channels (i.e.: Qi Kong) and it is possible to target the acupuncture meridians (acupuncture and acupressure), but it is not possible to isolate any part of the body. The body is a whole. So, to say that something "doesn't affect part X" is flawed reasoning. How do you know? There is no proof that yin yoga does not affect the acupuncture meridians! In fact, I would suggest quite the opposite is true.
While you can target a particular muscle through some intervention (for example, if you say, "I am targeting my triceps by doing pushups"), more than just that muscle is affected: you are working your deltoids, pectoralis, stressing the superficial fascia, deep fascia, the bones or the arms, etc. In the same way, I know of no way to isolate the sinew channels from the acupuncture meridians. To say that Qi Kong does not affect the meridians is clearly wrong. The work of Helene Langevin and others have shown some of the ways acupuncture affects the fascia which in turn affects the whole system of the body.
If the TCM informed yin yoga teachers want to broaden our knowledge and talk about the existence of sinew channels, that is great! Teach us! But to insist that yin yoga doesn't do something (like saying it doesn't affect the acupuncture meridians), more than a bold assertion is required. Cite your sources and prove your case. I suspect they can't prove their assertion because so much evidence exists that points to the opposite finding. (Plus, it is really hard to prove a negative! Can you prove there isn't a god? How would you prove that yin yoga does not affect the meridians?)