Would you mind going further into this one? Barring belief in dualism, I don't see how this can be possible.
The really short answer is that it boils down to a disagreement over what "free will" means. Compatibilists dispute the claim that free will is definitionally equivalent to something like a videogame with multiple endings, where your actions can change which endings you get (the "garden of forking paths" model of free will).
It's actually usually taken as an alternative to dualism, because if humans are 100% made of physical matter that's bound by the usual laws of physics, and we think something called "free will" is meaningful to talk about, then they must somehow be compatible. The alternatives are: 1. dualism, claim that some part of human thought/action takes place outside of the laws of physics; 2. deny that free will exists; or maybe 3. somehow deny that the laws of physics exist, or at least claim they exist in some different way than usually understood.
Daniel Dennett is probably the best-known-among-the-general-public proponent of compatibilism.
1) Rewrite it as: (fill in the blank) economic reasoning has few virtues in its own right, but is leveraged as a rationalization for policies convenient to (fill in the blank).
2) Ask yourself how much political power Keynesians have if they don't even have the power to impose Keynsian solutions.
3) Fill in the blanks for the statement in (1) as it actually applies for those with political power.
If you attempt to read it with the (perhaps generous) assumption that I'm not proposing a tautology, than you might see what I mean.
A Keynesian ideology provides justification for the use of state power that differing economic ideologies do not. It would seem rational (warning: assumption of self-interest) that, holding everything else constant, politicians have a tendency to endorse Keynesian thought.
Some number of my possible responses, although admittedly I would use none of these for Thiel's essay:
* Modern running shoes increase one's tendency to injury.
* Free will can exist in a fully deterministic universe.
* Saturated fat and cholesterol are not the primary causal factors behind heart disease.
* Keynesian economic reasoning has few virtues in its own right, but is leveraged as a rationalization for policies convenient to those with political power.