It is not P2P. The place where they most clearly state that as far as I can find is actually the documentation for their game SDK[1], but you can check that this is the case with netstat. In general it’s hard to get good voice or video chat with P2P connections because of the low upload of most residential internet connections, so few serious chat apps do it.

[1]: https://discord.com/developers/docs/game-sdk/networking

There's also this: https://blog.discord.com/how-discord-handles-two-and-half-mi...