DeFi means decentralized Finance, maybe the interviewer should have ask him what decentralized Finance is instead of DeFi? Might sound confusing using DeFi? I personally don't know what DeFi was until I figured it out that it was shorten name of decentralized Finance, I bet he knows what that is.
I'm skeptical about this. If he is the one who can get interviewed, he has to have certain understandings and fundamental knowledge about the cryptocurrency market. Lately, the word "Decentralized Finance" is like a hype on cryptocurrency forums. Obviously, when you know the word, you have to know its abbreviation! It doesn't make sense if he know "decentralized finance" but not know DeFi.