This may be off topic, as it's about aristotle's post, not about Barry's article, but hopefully will provide some background for many considerations.
google started treating it as a "sinkhole" for link juice
Wilburforce - Matt Cutts announced it back around 2009-2010.
aristotle, regarding the black holes, Matt was talking about what would happen to the internal PageRank "lost" via using nofollow when PageRank sculpting of internal
nav links. If you over-used nofollow on internal links after the change Matt announced, you would effectively lose useful internal PageRank.
I posted a relatively concise explanation of how I thought PR black holes worked in 2012, on this thread... Need some blunt advice for improving site Apr, 2012 https://www.webmasterworld.com/review/4441381.htm
Note that we used the astronomical term "black hole", rather than "sinkhole"...
I myself would not use rel="nofollow" on internal nav links.
Nofollowing is not at all the same thing as removing the links. Nofollowed links do count toward the denominator of the fractional part of link juice each outgoing link on a page can distribute, but the rel="nofollow" attribute blocks the distribution. Thus, nofollowed links are in effect a black hole for PageRank... not distributing the link juice but nevertheless diluting it.
Now, important distinction... that "lost" PageRank was about internal PageRank only, lost in nofollowed "internal" navigation links. Current kerfuffle has mainly to do with external links... ie, links from your site to external domains... though the change is inevitably going to affect internal linking as well.
I remember when the change was announced that I felt at the time that the change was going to mess up the eco-system of the web... but, at the same time, I thought that it was justified, as spam links on blogs and on Wikipedia were making those sites very difficult to maintain.
I should add that there's been a big confusion among many webmasters because of the change. They were either...
a) afraid to link out, for fear of being penalized...
b) they were being greedy and wouldn't link out, because they felt they'd be giving away PageRank to other sites (thus, they were called "PageRank hoarders")...
c) they used nofollow on all outbound links, for a combination of reasons (a) and (b).
The original idea of PageRank was that linking out, from pages and domains, would recirculate the PageRank supposed "lost", to other pages and to the web, and whatever was "lost" by linking out would be regained by natural inbound linking. Google, IMO, did, in fact, mess up the eco-system, because they did encourage PR hoarding... and it's hard to say how much they might have since Google compensated for the changes.
I think that Google is finally at the stage of making judgements independent of PageRank and associated linking signals, and perhaps would like to restore the web eco-system. I can make some guesses, but I don't know.
Here's our thread on the 2009 Matt Cutts announcement... Google Changes Treatment of PR 'Saved' by rel=nofollow Sculpting June 2009 https://www.webmasterworld.com/google/3925952.htm
Many changes have been made in Google's view of PR since. As an aside, I should note that the only really solid analysis of how PageRank works, or was intended to work, that I've been able to follow, was by Dixon Jones, a WebmasterWorld mod and formerly of Majestic.