I’m not sure, but I think any image can be used, and resizing is up to the consumer. For example, I just tested a small image on try, let’s see its Onebox:
https://try.discourse.org/t/testing-some-images-and-thumbnails-hint/1498
It is a bit confusing indeed, the |thumbnail logic is all on core, the theme component just requests that core generates additional thumbnail sizes, and uses them on the topic list. It happened to be the only topic (that I know of) with that piece of information, hence the quote.