"Isn't great" or "needs improvement" is quite different than "total clusterfuck". This last statement is extremely rude and dismissive of a lot of hard work that has brought great progress to the Linux desktop, which, contrary to the hand waving, is quite usable these days.
The quote isn't, "open-source software's usability could be perfected in a weekend". The quote doesn't suggest that it's easy, or not time-consuming, just that the end result right now isn't great.
Which I would agree with.
You're just saying that a lot of people are working hard to make it better.
Which I would also agree with.