The frontend is Node.js and Angular, according to their job postings: http://careers.stackoverflow.com/jobs/102502/front-end-devel...
It works like this: Jet buys the product from the merchant at the MFN price, then sells it to the customer for whatever price Jet chooses. Yes, the price could be higher, but probably will be lower than the merchant's (and therefore, Amazon's) price. They do this by sacrificing part of their commission the merchant pays Jet, and giving it to the customer instead.
They even have ways for the merchant to choose the same thing, for example, if the customer opts out of free returns, the merchant can pass a percentage along to the customer. Amazon can't get upset about that, because even though their MFN clause requires promotions to be consistent across platforms, there is a loophole in the case of Amazon tech stack not supporting the type of promotion being offered. Amazon has no way to rebate the customer for opting out of a free return.
Jet will deliver on its promise of lower cost shopping. Whether that substantially helps them unseat Amazon remains to be seen.
Is that why some vendors won't let you see prices until you add it to your cart?