yes, things like age/gender/location/etc are the things that aren’t private on reddit but mostly are on fediverse. your instance admin could see your IP and use GeoIP for location but that’s it, a simple VPN can solve that. and unlike reddit where they do targeted ads and resell data, fedi instance operators are mostly hobbyists and I doubt that really happens… if you’re really worried you could just use throwaway accounts with a vpn and remain private.