This section is where I basically stopped reading. Anything that earns interest is not risk-free. Full stop. The whole point of earning interest is to reward the lender (and to be clear, "staking" is lending to the system) for the assumption of the risk that they may not get their money back. There are low-risk opportunities (usually accompanied by low interest). And in the case of staking a token, the staking comes with the implicit risk that the value of the token may decline substantially.
I see a lot of talk of "risk-free" opportunities in the DeFi movement, and that's why I stay away. It sounds too good to be true, and it is.
There are other issues in that article -- in particular, most DeFi APY calculations are variable and constantly changing -- but that one was the full-stop for me.