Well, it turns out that Envato support officer had some confusion about which type of partnership I was talking about. I’m posting their clarification for anyone who is reading this thread:
When I said alongside, I meant two lanes to choose from. You can choose to purchase an Extended License and request the Authors permission. Or you can come to a private agreement, with the terms to be decided off-market. Usually a percent of any sale revenue.
Long story short; PSD or HTML extended license is NOT required if you want to convert that template to any CMS theme with author’s permission.