The posit is that the spoiler effect is bad: Fielding an additional extreme candidate on one side should not be helpful to the other side.
So with Instant Runoff elections, the majority gets their wish. The Green Candidate wins.
With Approval Voting, 60% of the people vote the Green Party Candidate, 40% of the people vote for the Republican Party Candidate, and 100% of the people vote for the Democratic Candidate. The Democratic candidate wins.
I don't see how this is a win for Approval Voting. The candidate that wins is a candidate that no one really wanted.
Well, clearly my example is rather contrived, but I suspect (perhaps out of ignorance) that any situation in which Instant Runoff would cause counter-intuitive results is also rather unlikely.
I'll definitely have to look at your reference, though. I do find such issues very interesting