It seems to be some kind of app you will have to install and use which will apparently put your BTC into a financial product they call as "covered calls". I am familiar with options but have no idea about covered calls.
These have been implemented using DLC (Discrete log contracts), which apparently enable smart-contract like or similar capabilities. (And why haven't we heard about this more in the forum is another thing).
Whatever the idea is, the fact that you install an "app" that manages your BTC just means that you are trusting the application and it will of course use your keys at some point in time. This is the same idea implemented by apps like Binance where you can "store" your BTC and they will allow you to earn yield. The blog is also unclear about where the "yield" comes from.
AFAIK with sovryn you are swapping BTC for some random token called SOV, and they put that into some smart contract. Doesn't seem amazing to me.
AFAIK with mintlayer you are swapping BTC for some random token called MLT, and they put that into some smart contract. Doesn't seem amazing to me.
Don't know about mintlayer but Sovry has been a sore disappointment till now. Of course a lot of early people made money when SOV pumped but that is about it. They are just another smart-contract platform and I have no idea why they want to show themselves as different from Ethereum. Its really the same thing with much worse UI/ UX. And to think that they got almost 200-300 BTC raised.