The point, as I read it, is that the your relationship's with an open-source maintainer as their downstream is very much unlike your relationship with any of your suppliers (or other suppliers, if you want to insist), because you haven’t negotiated a business relationship with them (and they may not necessarily even want one in the traditional sense). Thus it is, in fact, unhelpful to refer to them as “suppliers” in order to draw an analogy with your normal suppliers, because they can and will tell you to go take a hike if you treat them like one of those. And given the nature of open-source software and the amount of business leverage you have over them (none, if you go about it the way most commercial consumers of open-source code do), it’s not unreasonable of them to do so. (Distro maintainers, for example, need a very special set of social skills for handling ornery upstreams, even though they both receive more understanding for their demands, being volunteers themselves, and do actually have some leverage in the form of being the main form of getting your software in front of users.)
(This is the part where I disagree with the article: the important part is not that, legally, the usual license terms mean they can refuse to provide support; it’s that, socially, it’s not unreasonable of them to do so. Google and Facebook also have license terms like that, but people do argue it’s still bad of them.)
Perhaps looking into the analogy the term “software supply chain” is intended to evoke is more helpful. One possibility, which is what most engineering users of it (among others) intend, is, indeed, that your open-source dependencies are your liability the same way as your traditional supply chain is, and that’s true. Another, which is what the article objects to, is that you are therefore entitled to ask anything of them, and that’s false. Given that the liability and the entitlement are two equally important and mutually supporting sides of the standard supplier relationship, there is reason to dislike the an analogy that is only correct in half of its implications.
You’re free to disagree with any of this, to be clear, it’s just that you’ve said the article is only playing word games, while on my reading it does seem to have a point, as above.