The editorialized title says DRM but I believe "protected content" in this context means restricting the sharing and screenshotting of content in the app via normal measures, not that they applied tons of cryptography like more a mainstream meaning of DRM would.

DRM does not mean "tons of cryptography" and never did. It means anything restricting users' freedom to do what they want with the data.

No. DRM is meant to protect copyrights, but Telegram's intend was to protect private messages from being forwarded to the public, which has nothing to do with copyrights.

DRM is an instrument, which can protect anything. If you apply it elsewhere, it does not stop being DRM.

I see, thanks. So the title is totally misleading then.

Telegram has a ToS which blocks any client which does not comply with the same restrictions as the app. They have used it in the past to block apps which do not send the "seen" status on messages.