Great question! The short answer is that it’s not possible to use synonyms in order to return the exact same results across multiple unique search terms, but let me clarify further:
At this time, the Result Rankings feature operates independently of any Synonyms sets you might create. Synonyms allow you to define terms that are related to each other (so that they’ll be a part of the result response), but will not equate the terms exactly.
For example, say you have a synonym set comprised of three terms [apple, orange, banana] and each of those unique terms exists in a page result somewhere in your engine index. When a search is submitted for apple, though you will see results for orange & banana in the returned results, the results that are an exact match for apple will have a higher score/rank.
In order to have the exact same results return whether searching for apple, orange, or banana, you would still need to manually pin & curate the results for each term using the Result Rankings feature.