Are you certain? Not OP, but a huge chunk of early RFCs was about how to let giant IBM systems talk to everyone else, specifying everything from character sets (nearly universally “7-bit ASCII”) to end of line/message characters. Otherwise, IBM would’ve tried to make EBCDIC the default for everything.
For instance, consider FTP’s text mode, which was primarily a way to accidentally corrupt your download when you forgot to type “bin” first, but was also handy for getting human readable files from one incompatible system to another.
I had a pre-'@' email address and it was able to communicate all over the world.
My first reading was that you were disagreeing with the bits about email worrying about compatibility, and that part seemed reasonably true to me.
As to the other bits, I think even in the uucp era, email was mostly internal, by volume of mail sent, even though you could clearly talk to remote sites if everything was set up correctly. It was capable of being a worldwide communication system. I bet the local admins responsible for monitoring the telephone bill preferred to keep that in check, though.