I can't agree with this sentence because misguides the people about the true meaning of the concept homonym.
If we use the strict meaning of the concept homonyms, then "blue" and "blew" are not homonyms, and instead they are homophones because they have the same sound, but different writing. One good example of homonyms would be "sow" (the seeds) and "sow" (the female pig). In my opinion, the sentence would be more correct saying "are homophones".
Another good example of homonyms would be "lead" (to run something) and "lead" (the metal), nevertheless, "leek" and "leak" are another example of homophones .

Actually, these are "homonyms" by some definitions.

https://www.merriam-webster.com/dictionary/homonym

However, recently this word seems to be starting to have a more narrow meaning.

I know that it is a little confusing, but strictly speaking, homonyms are words written the same, however, I also checked many other pages, like yours, with an "open" concept for homonyms, but I can't agree with that because this open meaning will misguide terribly the understanding for most of the people. If we accept that, then we'll be in trouble to understand the real matter between homonyms and homophones.
My view:
homonyms = exactly the same spelling regardless of the pronunciation
homophones = exactly the same sound regardless of the orthography.
Thanks, CK.
Regards.

> homonyms = exactly the same spelling regardless of the pronunciation

I thought those were homographs. And homonyms, in the narrowest sense of this word, are both homographs and homophones (so they are both spelled the same and sound the same).

homonyms = homographs

Read this article
https://en.wikipedia.org/wiki/Homonym

Thanks for the link, that's exactly how I understood homonyms.

Broad sense - "spelled alike, but have different meanings." (so the same as homographs, although I'm not sure "spelled alike" = "spelled the same")

Narrow sense (which I usually prefer because it's useful to have this category):

" A more restrictive definition sees homonyms as words that are simultaneously homographs (words that share the same spelling, regardless of their pronunciation) and homophones (words that share the same pronunciation, regardless of their spelling) – that is to say they have identical pronunciation and spelling, whilst maintaining different meanings. "

There's an interesting video by Vsauce about those terms (homonyms, homographs, homophones, etc.):

https://www.youtube.com/watch?v=gTKeB8BnzPY

Anyway, I'm not really arguing with you. There are nerds and scientists with their "exact" definitions of words, and there are people who use them loosely, and that's absolutely fine.

I appreciate your words, however, my purpose it's to highlight the concept in the strict nuance of the word to avoid misunderstanding or confusion. Therefore, I believe that "blue" and "blew" are homophones (they sound the same) and not homonyms because they are not written in the same way.

May we ask the author to change the sentence?

In this time I disagree with you, CK.
Deniko posted this link
https://www.youtube.com/watch?v=gTKeB8BnzPY
and if you see the video is clear that "blue" and "blew" are heterographs and no homonyms, "- bank and bank (1) and skate and skate (2) are two good examples of pure homonyms, but not "blew" and "blue", however, if we use the broad concept of homonyms, -my favourite- sow (pig) and sow (seeds) are homonyms because they are written the same, but the pronunciation is different-" so, I don't pretend to set the rules of the use of the language, and specially with the English because I am not a native English speaker, but instead I prefer to help the people to clarify some blurred concepts with the use of the language that cause confusion and trouble to the most.