I think a much simpler and more elegant answer is "they didn't think of that, they just want to block Telegram."

As someone in Russia, the block pretty much only hinders use of the legitimate services, not of Telegram. Numerous services exist to allow it to continue, and the block is a hack-job to try to prove a point.

> WhatsApp and Messenger content. (emphasis mine)

Isn't WhatsApp supposed to be end-to-end encrypted making it next to impossible even for developers (Facebook Inc. in this case) to access the transmitted messages' content?

Wasn't it WhatsApp that sent a key back to Facebook servers? Technically it's end-to-end encrypted, but Facebook could decrypt it if they really wanted to.

Do you have a source for this claim?

It does not sound correct.

It's decrypted on my phone in their app. Theoretically, what stops them from connecting to my phone and reading the decrypted content from there?