While this is a real issue, the threat is best mitigated outside of the browser. In theory any application you run could put contents in your primary selection, the threat is what you do with that. The biggest threats I can imagine are insecure shell settings which the author pointed out and can be mitigated easily. Or as a commenter pointed out, cryptocurrency related activities could be at risk - such as pasting in an address to send the currency to could be hijacked and you probably wouldn’t even notice as the addresses are random. X is known to be insecure and if you’re doing something sensitive like handling cryptocurrency it would be best practice not to run X anyway.