What basically happening is that based on the prefilled email field value we are showing fields in a progressive manner.
I worked on very similar requirement just 2 days back. Initially, I also tried to use Forms 2.0 functionality of 'Known leads'. Forms 2.0 editor allows us to select to show either 'the form' or 'custom html' if the lead is 'Known'. When we select 'custom html', the 'form' is not included at all in the html generated. That's what I found in my testing.