That's not wrong. It's something that not everybody will like, but as long as the extensions don't conflict with anything specified by POSIX, that is permitted.
To me, the decision to let POSIX mode turn off extensions that are so widely available in other implementations that they have been approved for the next version of POSIX is an odd one. There is little reason not to already use those features today: using them is not going to seriously limit your portability. If you use them, you cannot also use pdpmake's POSIX mode. The obvious conclusion there, to me, is not to avoid using those features, but to avoid using pdpmake's POSIX mode.