The author, Daniel Greenfeld, is also coauthor of Two Scoops of Django (https://django.2scoops.org/) that has a lot of other awesome tips.

This is an interesting article but I don't understand why he says to forget the HTML and then goes into Flask.

His emphasis is that you as a web application developer shouldn't get distracted by writing HTML: use a framework which does the HTML for you, especially when it comes to producing forms and doing validation.

There's a form-building framework built into Django (called Forms), or you can use a different HTML/form-building framework such as Flask.

The take-home message is: building forms in templates is fraught with peril. Use a framework to save time and heartache.