Running "make -p" is running a full parser which someone else wrote. Grep is not extracting the targets from a makefile there, it's extracting the targets from an intermediate representation of a makefile.
> The real insanity with make is that it has no interface to check whether a Makefile contains a given target.
>
> Best you can do is to run make -n target to probe, but it's possible to write Makefiles that run code even though -n is used, which will defeat such probes at distribution scale. It quickly becomes an exercise in heuristics and output parsing.
The comment I replied to, chaosfactors, specifically specified grepping the makefile. You're arguing that you can find all the targets and I never said you can't. I simply said you can't find them all by only grepping the makefile.