You will find https:///www.wire.com/ interesting. e2e encrypted, clients for Windows, Mac, Android & iOS. And chats are synched between all your clients. Also .. no phone number required to sign up; use your email.
Also have a look at https://wire.com/legal/#privacy
https://app.wire.com offers full functionality on Linux.
That tweet you link to is over a year old. Wire is E2EE since March 2016. https://wire.com/privacy has security and privacy whitepapers.
PS. I work at Wire.
Do you know if it's open source or at least gone through an audit?
I don't know whether they have been formally audited.
This would require some kind of key exchange i.e. scanning bar codes.
I haven't looked into it at all, but one way I just thought of right now is to have your own devices p2p the message among themselves with original sender's information after the one used most recently receives it.