I find it odd that many of both the blog comments and the HN thread comments are focused on debating the merits of the feature itself rather than the broader point of the article, that a small isolated change made by one person can have a massive, long-lasting impact on software and on how billions of people interact with it for decades after it was implemented.
It’s like no one ever took a humanities class
This isn't necessarily true. The fact that we're still using squiggles decades later means that nobody else has come up with a better idea. If there are no better ideas, if this is the best UI design for spellchecking there is, then it's simple enough that, if not for him, someone else would've came up with it. How many ways are there to highlight a word in a sentence, really, especially when your constraint is that the UI for highlighting the word must not overlap with actual ways normal people highlight words in sentences because the application supports that for non-spellchecking features (highlighted background, color change, non-squiggly underline, etc.).