My argument is not logically inconsistent, but it may be a bit tricky to see why.
Firstly, I did not argue that quality is a pure function of potential profit. I said there's no profit motive to make desktop Linux better than it currently is (my view is that desktop Linux has been at a pretty constant level of quality for the last decade, which I'm sure some will argue with, but that's been my experience). That's a slightly different argument which I'll elaborate on in a moment.
Secondly, the fact that Macs ship with open source software doesn't invalidate my point. The popularity of MacOS X amongst developers can largely be summed up as "it's UNIX but with large parts replaced with proprietary components". What's left is whatever wasn't relevant to the consumer/pro-designer market segment, mostly a collection of command line tools. The other open source stuff (Apple's own code) was very much directed by the profit motive and open sourced secondarily, as a part of a product strategy.
There are two problems with the volunteer/subsidised-developer model that shine through when using desktop Linux. And in case you think I'm a clueless idiot who doesn't know what he's talking about, I've used desktop Linux for many years and even did contribute open source work to it about 15 years ago (I had patches in a few well known desktop projects). Eventually I became disillusioned because of these two main flaws:
1) Volunteers avoid boring work i.e. fixing bugs in other people's software.
2) Subsidised developers and volunteers lack any incentive to discard flawed ideologies and convictions
The second is the most important. The first problem can be addressed through Red Hat style cross subsidisation. The second results in massive, glaring weaknesses that developers rationalise as strengths rather than swim against established dogma.
Some examples of crap that is simply not tolerated in desktop operating systems built by people seeking profit but is/was often rationalised away as a strength in Linux: the audio server situation, completely messed up software distribution model, lack of support for proprietary kernel drivers, flaky backwards compatibility, general hostility to proprietary userspace apps, a bizarre insistence that anything important be written in C, refusal to work with any kind of content industry that wants DRM ... lots of policies that Linux users have just learned to accept as futile to argue with that Apple and Microsoft don't have the same hangups about. And the end result is, guess what, Macs just work a whole lot better.
I'm painting with a broad brush. The profit motive is just an incentive, a nudge, it's not an iron law. Apple and Microsoft have their own weird ideological hangups, especially in recent years, and they could really use a whole lot more competition. The Linux community has overcome entrenched attitudes a few times in the past - GNOME 2 and systemd are good examples of that. But overall the Linux community suffers far from more self-inflicted wounds that they're psychologically incapable of changing and without any financial incentive to do so, these things just fester for decades.