You should write that answer as a comment to the blog post. The author of the blog is very thorough and likely to take an interest to it, if there’s anything to it.
(As an aside, why are we assuming 80287 and not 8087? I know nothing about both, so it’s well likely that I missed obvious hints. EDIT: Ah, I guess because it’s the int 13 handler specifically.)
It was actually IRQ13 -- one of the input pins on the slave interrupt controller. It was typically mapped to INT 75h. I don't know if Windows remapped it.
(As an aside, why are we assuming 80287 and not 8087? I know nothing about both, so it’s well likely that I missed obvious hints. EDIT: Ah, I guess because it’s the int 13 handler specifically.)