I’ve literally wasted the last 20 minutes trying to remember the name of this website, and I failed! (I should really organize my bookmarks).
I really hope that they backed up date, but I fear they are using Unreal APIs to read data in real time.
EDIT: No, they are not in real time, this is positive
However, in the meanwhile I realized that you can still easily get the json they were talking about with Postman (Or even with a web browser, if you use a json beautifier).
I noticed too late that it’s just a regular get request with parameters 
.
This is my plugin for example:
https://www.unrealengine.com/marketplace/api/review/a00eb1e478ae4a8fb655aa22f9125b07/reviews/list
https://www.unrealengine.com/marketplace/api/review/a00eb1e478ae4a8fb655aa22f9125b07/questions/list
The product ID is the string that you find on the url of the seller portal
