At the very least they could've tested the current stable release of Opera - 10.62 - instead of 10.53.
To be fair, Chrome beta channel is still v6 series (6.0.472.59 beta). The dev channel is v7 series already though I believe.
Good question :) I didn't write the article, maybe ask @lucidchart on Twitter? According to the actual article title though, it seems the main interest was testing FF4 and IE9 against existing browsers, which would explain the absence of dev channel versions of Chrome.
Or more interestingly, they were looking at providing the best numbers to push their agenda the furthest.
As noted in a previous comment, a fair comparison would be to run a proper benchmark--which guarantees similar operating conditions.