Earlier quoted context omitted.
This is true, but it's also true that plain text emails do not prevent phishing.
No, but I think it's fair to say that they make it more difficult.
But it's also visible in the address bar of the browser after you click it... and people will fill in the form anyway.
Plain text doesn't solve complacency and distraction, it doesn't solve URL composition tricks and redirect wrappers, it doesn't solve spoofing, and it doesn't solve malicious attachments.
If Google's account emails were all plain text, I'm sure phishers would figure out how to make them look as identical to Google's emails as possible, and still get some people.
IMO we already know some good ways to mitigate phishing, but their implementation is spotty: DMARC, hardware token 2FA, and good IT practices in terms of patching, privilege, and backups.