Why? Factually they are...

Case in point.. I used to run game servers for maybe over 1000+ guilds over the years I was active. Even talking 10 years ago, people were still using Skype for connecting to games... just because it doesn't fit your use-case doesn't mean the market wasn't competing.

IRC was also in the space. Look at the dozens+ IRC servers which had communities built around them... they've mostly migrated to Discord.

I'd love to know your counter point to such an absolute-claim though... because I positively agree with the author.