GNU coreutils deviate from POSIX in a number of instances.
GNU coreutils are also verbose and potentially overengineered. Compare GNU ls.c (4726 LOC) to FreeBSD ls.c (1030 LOC), for example. Even if you include the other BSD source files used in ls, the FreeBSD version is still half the size of the GNU version. GNU grep is over 3000 LOC, whereas the FreeBSD version is 724.
GNU and Linux won. The only BSD environment is macOS, and if you put GNU on macOS, then the only thing I can think of that's worth a mention is alpine and busybox containers, but at that point it feels like we're just grasping at straws.
Well, I do :) That's why I said I was only speaking for myself.
As long as we have a rich variety of free and open source software to choose from, I think we've all won.
Where are you getting that number from? Because I have a bit over 1,500 lines in usr.bin/grep (code only, excluding comments and blanks) vs. 3,900 for GNU grep.
Also you can't compile bsdgrep with just POSIX libc (such as musl) since it relies on some extensions (specifically: REG_STARTEND). So if you're looking for POSIX_ME_HARDER then bsdgrep isn't actually the right place.
The number of lines is obviously version-dependent. Looking at the file mentioned above on a FreeBSD system to which I happen to be connected, I see 805 lines, including the comments and blanks, while all the source files total 2027 lines, also including the comments and blanks.
To be fair, one of these two flags comes built in with GNU ls, but using GNU ls comes with the downside of losing support for macOS extended attributes (and GNU ls doesn't have --group-dots-extra-first).
It is not my responsibility to upstream anything. If you feel like it should be upstreamed, my additions are open source. Go for it.
If anyone is curious, this is the resulting Perl oneliner:
perl -WE 'printf ("COMMAND;DESCRIPTION;USAGE\n");while (my $line = <>) { $line =~ /^(.*):\s#\s(.*)\[Usage\s`(.\*)`]/ && printf("$1;$2;$3\n")};' Makefile | column -c "3" -s ";" -t