Yes definitely it could be because of risk. There is also a risk that the GUSD isn't returned (with Gemini it's not actually defi, there is no smart contract mandating its return, there is counterparty risk). I'm not saying it's 8% risk free when the risk free rate is 0%. But the article indicates that there is an actual inefficiency here due to institutional restrictions on cryptocurrencies and an institutional demand for products that approximate them.