I don't know who is right here; the article linked above doesn't explicitly say when the request to load the external content is made. At first glance, it doesn't make sense for Google to preload all external content, so the latter approach (loading it when the email is opened -- which might never happen) makes the most sense. On the other hand, Google could use this to speed up their email interface, in which case they could choose to preload some content (such as static images) when the email arrives.

There are 1,000 ways that Google could implement this, and it seems that marketers are scrambling to figure out exactly what their hits really mean.