I know that the canned rejection reason is indeed rather unspecific, but I also don't see much use of this tag.
Tags are mostly there for the askers. We expect an asker to be able to add the appropriate tag to his question. We also expect askers to search for their problem before asking a new question, and if their keywords match tags, the system is able to offer better matches than if they match body text. I am not saying that this happens every time, but the system should be geared to support these scenarios for those who actually make use of them.
Enzymatic browning is something most askers don't know about. It is not impossible that somebody comes in and asks a question about "what are the rates of enzymatic browning in Linda potatoes at 20 Celsius" or similar, but I don't remember ever seeing such a question on the site. What we see is usually something like "why does the color get funny?", like in the question you linked. The term is contained in the answer, not in the question.
I would argue that such terms are bad candidates for tags. Those who are supposed to add them to the question won't do it - if they knew the term, they wouldn't be asking the question in the first place. Those who may think of adding a "browning" tag to their question will get the suggestion "enzymatic-browning" from the system - but if they don't know the difference between enzymatic browning and other types of browning, they are likely to use the tag where it doesn't apply, reducing the findability of our information.
In short, it seems to me that it is not really a good term to add to the tags.
I cannot of course say if Divi's reasoning to reject it was based on the same arguments. Maybe she had some different ideas about the reason to reject. And maybe others prefer us to have the tag. I am just adding my opinion on why it is probably not very useful to add.