I’m aware that you can add a word to the dictionary, that’s not the issue.
The irritating thing is that any unknown word is marked as misspelled, which is simply incorrect, there’s just a chance that it may be incorrect, but it’s definitely true that it’s unknown to the spell checker. Therefore I want to suggest changing the text to “unknown word”.
“Walt” may be unknown to the spell checker, but that doesn’t make it misspelled. And depending on how often I exchange email with someone named Walt I may opt to add it to my dictionary by way of the clearly available option.