It's backwards compatible: software compiled on centos 6 will be using the older memcpy@GLIBC_2.2.5 symbol, which still exists on newer glibc together with the current memcpy@GLIBC_2.14 symbol, so it will work. What doesn't work is compiling with a newer glibc and expecting it to work on an older system.

The example above is actually a great example of bending over backwards to keep compatibility with broken userspace. Some programs incorrectly called memcpy with overlapping inputs, and an optimized version of memcpy started breaking these programs. Instead of just letting them break, the older symbol was kept with a slower implementation which accepts overlapping inputs, while new programs get the faster implementation at the memcpy@GLIBC_2.14 symbol.