He means more that some software is not shared (or available to) with everyone. Systemd is very explicit in that only Linux is supported. I guess pulseaudio is also a problem.
A large part of open source is to share the source code so that others can take that work and do their own cool things / innovate. But then it has to be available.
Yes but isn't that a core problem, if you want to make the best use of something (in this case Linux) you will want take advantage of all the features it offers.
From what I gather the problem is that other systems (in this case *BSD) doesn't support certain features either because they don't mesh well with the BSD system(s) and/or has a lack of manpower to implement them.
A large part of open source is to share the source code so that others can take that work and do their own cool things / innovate. But then it has to be available.