Some anecdata: I had the flu NUMEROUS times as a child/young adult. I know what the flu feels like. A couple of years ago, I got the flu (no, I didn't go to the doctor because I knew it was 'just' the flu). I was feeling crummy one day, and the next I was curled up in bed, unable to move, fever with body aches, and slept most of that day. My wife told me about elderberry and gave me 4 or 5 spoonfuls of an elderberry tincture throughout the day. The next day, I was shoveling compost in the yard.
“We found no evidence that elderberry benefits the duration or severity of influenza. Post hoc analysis suggested primary outcomes with elderberry taken alone (without oseltamivir) were 2 days worse than with placebo taken alone. Our results contradict previous studies and demonstrate the need for further studies.”
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.
Some anecdata: I had the flu NUMEROUS times as a child/young adult. I know what the flu feels like. A couple of years ago, I got the flu (no, I didn't go to the doctor because I knew it was 'just' the flu). I was feeling crummy one day, and the next I was curled up in bed, unable to move, fever with body aches, and slept most of that day. My wife told me about elderberry and gave me 4 or 5 spoonfuls of an elderberry tincture throughout the day. The next day, I was shoveling compost in the yard.
https://pubmed.ncbi.nlm.nih.gov/15080016/