"The 501(c)(6) differs from the more familiar 501(c)(3) designation in that we are not a charity. The 501(c)(3) is explicitly designed for charitable organizations, and confers the additional benefit of donations being tax-deductible. Over time, though, the definition of a 501(c)(3) has become extremely distorted, especially in the software space, since companies were able to convince the IRS that making open-source software is a charitable/scientific activity. The result is that large companies were able to fund their own development by creating a “charity”, open-sourcing some of their core technology, and then building their extremely lucrative closed-source software on top. That way they get to deduct the core tech expenses from their taxes! What a deal!"

I get that, but I don't understand why it supports a 501(6) in this case[1].

Just because others have abused it doesn’t mean you should give up on it. Even if it's only about sending the right signal, that still matters.

Or is this about brutal honesty and they are saying bluntly: We're not a charity, so don't expect us to act like one in the first place.

If it is that, then why would anyone support them apart from their sponsoring organizations?

EDIT: Reading the whole thing carefully, I think they are going for an exclusive club. I genuinely wish them well, but to me it looks like a quite quixotic endeavour.

[1] There are many cases where a 501(6) makes sense. I'm strictly arguing the "Handmade Software Foundation" case here. Otherwise it gets complicated quickly.