"Yes, but when he says 'Sweden works, Cuba doesn't, therefore free market economies are better' his logic is hard to follow. "

But he does not say that. You are projecting your own prejudice into his quote. Therefore, and the words that follow, are yours.
I agree that Cuba is also restricted and that it plays an important part, so that it's not a clean experiment. I would argue, though, that the burden of the proof is on he who makes the claim that full equality is indeed the system maximising the condition of the worst-off in the long run.
All Krugman is saying is that there are limits to redistribution -in fact, let me quote the very previous sentence: "And before the trolls jump in to say aha, Krugman concedes the truth of supply-side economics, that's not an argument against progressive taxation and the welfare state; it's just an argument that says that there are limits."

Krugman's mention of Cuba and Sweden merely indicates that there is no obvious logical argument to reject even full equality, merely empirical ones. And he very clearly did not pick a particularly neo-liberal country as the working one.

I was going to have one chapter in this series about our embracing evidence-based thought rather than slogans and demands of ideological purity. Are you telling me that this is not a shared trait here and that you reject it? If not, what do you fault him for?

"Is it hard to imagine that there might be people in Cuba who are better off than people in the US? "
Is it hard to notice that this is a shockingly unfair point? Comparing one's maximum to the other's minimum?

"The subtext is an obvious and rather naive or unquestioning acceptance of free market exceptionalism compared to the alternatives"
Really? From someone who keeps singing the praise of social democracy and who picked Sweden as the example of a country that works, Sweden that, as you yourself said, cannot be called a free market economy?

"Free market economies can only work if they're based on an imperial model which steals resources from other countries."
This may be true, but is rather too flippant a statement to be accepted without a convincing demonstration. Otherwise it's just a slogan.

"That's a strange definition of 'work' - unless you start from the assumption that they're the best of all possible arrangements by definition, and work back from that."
Again, Krugman has written hundreds of pages slamming the direction USA were taking. Why do you project the neo-liberal agenda onto him? I can think of no reason other than the idea that anyone who does not concede the whole Marxist doctrine is an opponent to be fought.
Surely that is not your reason -but then I am missing it entirely.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi