They probably aren't the friends people are thinking of when referring to things like this. The benefit of friends isn't just that you have someone fun to talk to, it's that you're building out a social support circle. Your discord friends can't come over and help you clean up after a flood, or watch your dogs while you're away on a sudden emergency, or cook you a meal when you're grieving a loss, or help you get an interview at a job shortly after you're fired (or at least, not one local to you).

Loneliness is a big topic now, imo, because people are losing helpful human friends and relying on middling digital friends. Just like how looking at pictures of a forest is nowhere near as healthy as actually going to a forest.