What he means ( From my own experience, as an old time Gentoo developer ) is that if a project ships a default Makefile (configure.ac or whatever build system they like) that includes -Werror, they are inviting for a huge headache downstream.
There has been plenty of times where I've been forced to mangle Makefiles because the developer had -Wall -Werror in their Makefiles, and built with an older/different/odder version of GCC/compiler-of-choice than what we did, and thereby causing a ton of failures.
Compilers become pickier. When you release a piece of software to the wild, -Wall and -Werror do not belong in the sources, as two months later, your tarball will fail to build against the current cvs version of GCC, because it suddenly warns in -Wall that you don't have your commas aligned vertically with your paranthesis, so the smileys turn the wrong way.
( Or similar . )
The point is, be strict when debugging and developing, be lenient to downstream, don't force downstream to patch Makefiles, configure files and similar just because you ship with -Wall -Werror -D_FAIL_ON_GLIBC22