If you believe the statement "if someone is promising you consistent above-market returns it's either a scam or there is unknown or undisclosed risk" it might be true that you don't understand DeFi to some degree. DeFi isn't a single market, it's millions of micro markets that are accessible through what amounts to a single API.
So when you have millions of markets with different returns that can be traded in every imaginable way (and some you probably haven't imagined), throw in an insane amount of dumb money, people willing to borrow at high interest rates (relative to the real world), and a laundry list of factors that introduce inefficiencies into the market, it's quite easy to find pockets of above-average returns if you're smart. I have no idea if Stablegains was actually smart, but it's more than possible to achieve above-market gains in DeFi without exposing yourself to outsized risks.