How can a collision attack using MD5 be used to break WOTS
I believe that, in the question you cited, the answer included:

BTW: this appears to be more about the proof technique used to prove W-OTS, rather than the actual security; we can create MD5 collisions (invalidating the proof), however we don't know how to use those collisions to actually attack W-OTS-MD5; hence it would appear to be secure (but we can't prove it).

That is, we don't know how a collision attack would break Winternitz; we just can't prove that it can't.