You're correct. Parent comment says something true (some / many characters don't have a representation in most fonts), but the inference it makes is wrong (this is a bottleneck for communication)
And specifically these characters that are not present in mainstream fonts are getting so far down the long tail that most natives wouldn't know how to write them, so would result in e.g. hiragana in Japanese if they knew the word at all. To some extent computers have even increased the variety of kanji usage as you'll get textbooks that describe a word as "usually written in kana" because it's infrequent enough that an average Japanese person wouldn't be confident to handwrite it from scratch, and so twenty years ago that meant they had to use Hiragana.
But in these days of computer IMEs when they type it in phonetically and get a list of options presented to them, they're confident enough to pick one, and so the kanji gets used a lot more.
so far down the long tail that most natives wouldn't know how to write them
True, but if we can't even display them then chunks of language are going to effectively disappear because people will have less and less opportunity to encounter them.
