That essay was written about 5 years before Trello was spun out of Fog Creek. You can see Joel in the photo on the WSJ article covering that story: http://blogs.wsj.com/venturecapital/2014/07/24/digital-white...

To connect the dots, it's overwhelmingly likely that Atlassian was in fact "the Australians" referred to and Atlassian's tremendous growth (which was foreshadowed in Joel's piece) is why they could by Trello 7 years later.

It's interesting to look at in JOS in retrospect because he literally blogged the birth of company A, its competition with company B and it's spin-off C, which B later bought.