I think the Seifer = Malfoy speech mostly comes from the fact that both characters start their respective stories with the possibility of moving on to be bigger and better things and both end up becoming minor characters in the end. Both are under utilized, and both have pretty unsatifactory development and endings.

Seifer was a waste of a good character who could have been a real force in the plot and gets relegated to mindless flunky; Malfoy is a whiny, spoiled brat, but he was also being geared up for bigger things that never came to pass. Basically, the analogy comes from this and less of them being similar characters, that original thread was mostly silly spam than an actual true character analysis.