I absolutely agree with the central message. I would add a few reasons, notably lack of end-to-end encryption and partial ownership by Tencent, as strong reasons to stay away if you can avoid it.

That said, a lot of the particular details listed here actually aren't right to me.

• By choosing Discord, you also lock out users with accessibility needs, for whom the proprietary Discord client is often a nightmare to use. - From the footnote: "What I can tell you is that, to my surprise, Discord’s accessibility has apparently improved in recent years, and more blind people are using it now. One of my blind friends told me that most Discord functionality is very accessible and several blind communities are using it."
• Users of novel or unusual operating systems or devices (i.e. innovators and early adopters) are also locked out of the client until Discord sees fit to port it to their platform. - What do you mean? The web client doesn't work on web browsers that are running on unusual OSs?
• Discord also declines service to users in countries under US sanctions, such as Iran. Privacy-concious users will think twice before using Discord to participate in your project, or will be denied outright if they rely on Tor or VPNs. - I've used Discord through Tor and as far as I can tell it works fine.

Like I say I actually agree with the central thesis but not with more or less any of the specific reasons he cites.