Producing cache friendly output is how you scale easily.
A content management system that doesn’t produce cacheable html isn’t scalable and one that does is. It’s not the job of the content management system to serve the cached pages. That’s what CDNs, browsers, and caching layers in general are for.
Caching is one facet of scaling (albeit a large one). It's not necessarily the job of a CMS to serve cached pages but there's a bunch of other considerations in scaling, particular where database writes are concerned; something that will be relevant in a system where users post content.
Also, see the final paragraph of the comment above for discussion of the extent to which WP "produces cachable html"
The gp isn't "wrong": putting Varnish in front of WP is a possible solution to the fact WP doesn't scale, not a disproof of the fact.
Furthermore it's a highly limited solution: WP is only static if you limit it's use to its static features, and configuring Varnish for the unholy mess of 3rd-party dynamic/interactive/form-handling WP plugins is nightmare territory.
Wordpress doesn't scale.