"Sed" is the name of a specific tool. It is not defined by the GNU tools, but has existed in some form since 1974, well before Perl. GNU sed and POSIX sed both support BRE and EREs, but not PCREs.
Maybe there's some other implementation of sed that supports PCREs but that would really be an extension of that implementation of sed rather than a property of sed.
And maybe there's some GNU tool that uses PCREs, but that GNU tool would not be GNU sed, so it would not be a relevant property.
Anyway, they probably should have said BREs or EREs rather than "sed"...
Maybe there's some other implementation of sed that supports PCREs but that would really be an extension of that implementation of sed rather than a property of sed.
And maybe there's some GNU tool that uses PCREs, but that GNU tool would not be GNU sed, so it would not be a relevant property.
Anyway, they probably should have said BREs or EREs rather than "sed"...