I will echo those who have said, and those who will continue to agree, that adding an observation as unknown is better than not adding one at all. Personally, unknowns don’t really bother me. However, there are also a few practical considerations here that may lead to comments about adding IDs (I’ve left some myself, always intended as well-meaning advice and not criticism):
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Many unknowns are uploaded accidentally. I’ve often added a broad ID along with a comment explaining why doing so can be helpful and caught the eye of the uploader, who realized that they had simply forgotten/overlooked adding the ID themselves (often a much more specific ID than I was capable of adding myself). As a side note, I tend to include the comment explaining why adding a broad ID can be helpful because it avoids complaints like “well duh, I knew it was a plant” (in which case, why was it left unknown?)
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As already noted by others, in most cases adding even a broad category like “bird” or “plant” can help the observation get identified much quicker. This is why the “Frequently Used Responses” page on iNat has pre-prepared comments addressing how to add an ID (https://www.inaturalist.org/pages/responses). I wonder whether the paragraph mentioned in the OP was one of these? If so, it’s worth noting their primary purpose, as noted on the page:
It’s preferable only to use these comments on observations by new users, as there are many reasons why an experienced user may not add an ID to their observations.