James Arthur believes that the free market is perfect; perhaps not absolutely prefect, but better than any market distorted by any kind of human - government - intervention.
Although he doesn't seem to realise this, his delusion is based on a useful simplification devised by economic modellers back when computors were human beings with slide rules. The modellers reasoned that if the market was perfect, it would be mathematically tractable, and proceeded to generate economic models based on this assumption, which worked tolerably well, for the right kind of problems.
This is exactly equivalent to the way we use the idea of a perfect op amp in roughing out a circuit - the perfect op amp has infinite gain, so the closed loop gain of the op amp stage is entirely determined by resistor ratios (impedance ratios for feedback networks involving capacitors and inductors) and no delay, so it won't oscillate.
We all know that op amps aren't perfect, so the next stage of the design involves figuring out the effects of the imperfections of the real op amp that looks like it suits the job, and changing the circuit to take these effects into account.
James Arthur doesn't do practical economics, so he doesn't have any grasp of the imperfections of real markets, and he has some kind of problem accepting Keynes' insights into these imperfection, and Keynes strategies for getting around them.
Presumably - like most right-wingers - he has confused government interventions aimed at taming the weaknesses of real markets, roughly equivalent to the frequency compensation capacitors that cut back the high frequency gain of real op amps - with the communist system of central planning, which solves the problem of delayed feedback in real markets by going over to feed-forward control which works - after a fashion - but doesn't do the kind of self-optimisation on-the-fly that real markets can manage if you can tame their natural tendency to do boom and bust or collapse into self-serving monopolies.
Because central planning didn't work well, he believes that any kind of government intervention is equally flawed, throwing out the Keynesian baby with the communist bath-water.
Oddly enough, he doesn't seem to see any problem with the conceptually similar anti-trust legislation that is aimed at preventing monopolies
but he can't see that the natural tendency of traders to keep their money in the bank when the economy is in recession - and profitable business opportunities are rare - is equally unhelpful.