I believe the "x86 monopoly" was meant to refere to how only Intel and AMD are legally allowed to make x86 chips due to patents. X86 is currently a duopoly, and if Intel and AMD were to merge, that would become a monopoly.
> intended so they're not making an effective argument
To be fair I'm really struggling to somehow connect the "x86 monopoly in the late 80s" with the remainder of their comment (which certainly makes sense).
By that standard if we exclude mobile x86 has a much stronger monopoly these days than in 1985. Unless we exclude low end PCs like Apple II and Commodore 64.
In 1990 x86 had ~80%, Apple ~7%, Amiga ~4% (with the remainder going to lowend or niche PCs) so again not that different than today.