His point is caching of http or https traffic in the hope of speeding it up for local access is pretty pointless. Unless your doing mitm on your https, there is nothing to cache. And http is pretty much gone anyway - and even if just http, its dynamic generated sites.. There is really nothing to actually cache.
The browser caches stuff anyway that can be cached - since its on the end of the https tunnel..
Other than filtering traffic, there isn't much to thinking your going to cache all that much data with your proxy these days.
Its not like the client is going to be downloading the 20KB logo image of some website 100 times a day.. The browser will cache that, etc.