I expect you’re kidding, because it’s hard to believe someone made it onto Lemmy without knowing this, but here goes anyway:

They never said “don’t decrypt client side”.

Lots of things you probably use every day is end-to-end encryption.

HTTPS in your browser uses TLS to ensure that the content you receive is encrypted on its way to you and that it hasn’t been tampered with on the way (confidentiality and integrity).