HN filters some combining characters? That's weird, compared to the symbol/emoji blocking.
Also I'm reminded that the unicode normalization annex suggests that legitimate grapheme clusters will be 31 code points or less. "The value of 30 is chosen to be significantly beyond what is required for any linguistic or technical usage."
If I had to guess, they probably filtered the ones that could be used to break page layouts by creating very-tall glyphs.
I guess that's one way to do it. Pretty far from ideal though.