I do understand the desire to improve the speed of indexing books to make the site useful.
That said, the decision not to index those ingredients means that data is lost forever to the site. If it were in the database, you could always have a setting in the frontend that filtered out that info if it was distracting. Also, if you had that data then you wouldn't need indexers to indicate whether a recipe was, say, vegan. It could be generated on the fly based on the database info. Likewise, 'glutenfree' could be done based on ingredients so that even recipes that didn't specify they were such or that the indexer didn't catch would come up in the search.
I can think of situations where some of these ingredients might searched for. For instance, someone might be interested in a savory drink or smoothie that contains garlic. No way to search for that unless the recipe calls for enough to trigger it to be listed. Or maybe I want to find some ice cream recipes or protein drinks specifically with eggs. And then there is possible functionality for searches excluding ingredients: Flourless chocolate cakes, eggless ice creams, whether or not the book called them out for this status and the indexer put them in the egg-free category.
But I understand there are competing interests here: the site also needs lots of books to be indexed to be useful in the first place, so speed of indexing does matter.