Only problem is that big blockers realizes that even 20MB blocks cannot accommodate the transaction volume they speak of
Indeed, although to be fair, it seems likely that 20MB was deliberately chosen because Hearn and Andresen knew it would be rejected. This set them up to appear like they were interested in compromise, whereas the reality is that this whole blocksize debate was began so that they could get their hands on the keys to Bitcoin's source code.
nor does it solve 10min confirmation time. So other layers must be implemented which cannot (efficiently) be done without segwit. So they're just trying to hold segwit hostage.
Yep. Although I wouldn't rule out other ways of solving signature malleability, or other ways of implementing 2ndary protocol layers.