You didn't know whether it was just the flu. You don't know what your results would be otherwise. Don't give advice based on n=1 where you don't even know the disease.
You can have flu you barely notice as well as common cold that keeps you semi-conscious for a couple of days. Unless you get swabbed, you just can't know.
It's also not uncommon to have a "24 hour flu", which aren't actually caused by flu viruses, but result in flu-like symptoms for roughly a day, then a rapid recovery. Sounds like that was the case for OP, and I doubt elderberry had anything to do with it.
You can have flu you barely notice as well as common cold that keeps you semi-conscious for a couple of days. Unless you get swabbed, you just can't know.