I think authors argument is not supported by their examples since in this day and age for the purposes exemplified we use messaging services like Slack etc in place of email. Email remains for many automated messages where you probably want to be able to quickly scan through information - login info, password reset and so on - so styling it is really helpful.
Sure, but what about embedded images?
You can continue to use them. Drew is advocating for use of multipart emails, which contain a text/html and text/plain component.
For my site, we just take the html and strip all the html tags out. Surprisingly it works pretty well.