It's entirely possible that they obtained your IP address via social engineering too.

A prime example of this happening is League of Legends. Game is run on dedicated servers, but people still get DDoS'd. In that instance it is a reverse search of your Skype handle which exposes your IP. Riot even has a guide regarding it.

https://support.riotgames.com/hc/en-us/articles/201751764-DDoS-Prevention-Guide

It's also possible that your IP was obtained somehow via a first party service or VOIP. The latter is a staple of online gaming has resulted in a number of DDoS attacks on players in games like Rust, Ark, CSGO, etc. Some DDoS attacks even focus the game servers themselves.

Your only personal defenses are having your ISP change your IP or investing in a VPN that can take the flood bandwidth.

I suppose you could also ask game developers to strip all P2P aspects, but I've never known a game to host servers for comms. Happy to learn of those if so.