I think he mis-states the case. An overall deflation slows the economy primarily because wages are notoriously sticky on the downside. It’s far too difficult to get the existing labor force to accept individual pay cuts, so instead businesses facing lower sales (due to deflation) cut workers instead of wages.
This leaves the ones still working better off because they still have pre-deflation wages to buy deflated goods, but the layoffs drive employment down aggravating, or prolonging, what is already usually a monetary-induced recession.
The last time we experienced this we had the Great Depression. We really don’t want to go there again.
A drop in oil prices is good because it frees up personal resources, but the spending of those resources on other goods will drive up the prices of those goods, offsetting the effect on overall prices. It’s not deflation, but a repricing of goods relative to one another.
Leftists are bemoaning low prices for the same reason that they supported anything that raised prices -
they want “the masses” to use less of “their” resources.
Look at the pre-FED deflations in the US. You don’t see the problem you’re describing.