They're talking about dollars

The obvious and charitable read of their post is that they use "memory" to mean main memory/RAM, "CPU cache" to mean CPU cache, and "memory is cheap and plentiful" to mean "memory has a low dollar cost and most systems have a whole lot of it".

Your read -- that they use "memory" to mean "anything which stores bytes, including caches", and that they use "memory is cheap" to mean "accessing RAM has a low latency" -- is uncharitable, contrived and even literally self-contradictory ("CPU cache is memory" + "memory is expensive to access while CPU cache is cheap to access" is a contradiction which can only be resolved through using different definitions of "memory" in those two sentences).