Though I don't understand these arcane theories about probability

Well, which part of the following is so difficult to understand? I thought you were big on phlosophy

To Keynes, probability is a branch of logic: the theory of rational thought under uncertainty. Ordinary logic is just the subset of rational thought dealing with certain (or certainly false) propositions. I think this is a really interesting approach. Probability to Keynes is relative but not subjective. That is, probability is always relative to some data (or hypotheses), and so it is in a way subjective since each of us has different data/knowledge/experience, even different mental acuity. However, Keynes' probability is not subjective in the sense that a correctly formed probabilistic reasoning, being enunciated relative to explicit hypotheses, should be valid independently. Keynes writes at length about the problem of induction (reasoning from particular, though possibly numerous, observations to general statements) and he stresses that, contrary to what has been asseted by philosophers in the past, the fact that an inductive conclusion turns out to be false does not invalidate the inductive reasoning relative to the information available at the time the conclusion was formulated.

That's all the diary says about Keynes' theory of probability. The rest is not about probability proper.

Kupiec points out that, in sum,

There is no qualitative difference between determinism and probabilism.  (P. 36)

Or, an event with a probabilty of "1" is qualitatively a probabilistic event in exactly the same sense as an event with a probabilty of greater than zero but less than one.

I also don't know how that contradicts Keynes, though I insist I have not gone into any detail into the details of what Keynes said.

As to the rest, extraordinary claims require extraordinary evidence

I think he's on his way to becoming as famous and important for his work as is Charles Darwin for his and is perhaps rightly to be seen as the contemporary who has best understood (and restored) a proper grasp of Darwin's place and importance.

If you are not convinced, try it on someone who has not been entirely debauched by economics. — Piero Sraffa