Synonyms really meant additional words around your keywords, and not a brand new keyword that has the same or close meaning as your keyword. For example, keyword "kitten" wont get you "baby cat" but will get you "adopt kittens".

https://www.en.adwords-community.com/t5/Improve-results/Synonyms-does-not-new-words-but-words-around-keywords/m-p/91162#U91162