Hey Grant,
Looking at that article, I would be suprised if progressive profiling worked out of the box. It looks like it's not really using a Marketo form but doing a dual submit to Marketo from the Unbounce form.
If you look at this article you will see Breno has a suggestion for a "pseudo" progressive profiling for use with forms on non-Marketo pages that could be useful: https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PnMIAA0
Essentially doing another API call to retrieve lead details, then dynamically show/hide fields based on what you have.
Integrating that within the Unbounce framework could have other challenges but seems doable in theory.