I think it’s useful to document the reason for the policy (so that any future proposed revisions to the policy understand what the goal was), although I concur with @maflcko that unstable external links are problematic for long-term preservation. I think this could be handled by including a brief summary of the reasoning inline (sufficient to grok what the point of the policy is), perhaps including an external link for attribution purposes.
The actual content of the “rant” doesn’t appear unprofessional to me; it’s pretty much exclusively composed of technically coherent points. I suspect that the word “rant” in the description is intended either in self-deprecating jest, or as a reference to the motivation of writing the document (I assume the author intended to cite this document when criticizing various specific FLOSS projects’ practices on the subject). The document does seem a lot more thorough of a rationale for the policy than I had seen elsewhere (it raised some minor points that I wasn’t aware of when I filed the issue).
I would have no issue with linking to that external document, as long as sufficient summary is included inline that a potential loss of the external link doesn’t cause undue confusion. At the very least, including a link for context is better than providing no context at all: if the link dies, the dead link is unlikely to provide negative value. I think it’s also good-neighbor behavior to attribute documents that influenced the policy; I certainly wouldn’t want people to omit attribution to my work just because they’re worried that the link might become inaccessible in the future.