Linus has always been pretty adamant about not breaking API behaviour even undocumented ones. But in this case, undefined behaviour had been previously documented.
Also, was it him or Ulrich Drepper who were against changing memcpy undocumented behaviour. (mempcy used to work with overlapping regions too.)
PS. This mailing thread is from 2010. It's really old.
Linus took extra steps to no break autofs behavior, even though it was (more) due to a bug in GCC than anything else.