Most DEXs are built on some form of smart contracts on some specific chain which inherently makes them open source. So, I assume that it is technically possible to create an API that will tap into that smart contract and extract data in real-time.  But, just because it's technically possible doesn't mean it's easy or guaranteed. If this alt coin startup is dodging the question about how they're getting real-time data, it raises red flags. So I wouldn't rule out BS just yet.