Varnish is much more flexible than nginx. If you need to do complicated things around caching (and many people do), then it may ultimately be easier to cache with Varnish than nginx. Of course, many people don't, and hence nginx's built-in caching makes more sense. It's a purpose-built tool vs. a generic tool type of argument.

The article doesn't go into the reasons why, but they alluded to doing something interesting with caching:

> By flipping this standard use case around, using the right VCL configuration, and relying on some advanced Varnish features, we can address all three problems discussed in the previous section.