So it's that simple, huh? The only problem with that logic is that GT5 was not a system seller then either, and we all know that's not true. Going by NPD figures (the only reliable monthly figures we have for that period, except for Japanese sales, but GT5 flopped over there), PS3 saw a normal October to November increase (roughly double the October figures, which is expected for November across the board) in November 2010, the month that GT5 launched in (250k -> 530k). Furthermore, PS3 November 2010 sales were lower than its November 2009 sales (530k -> 710.4k), and that was a year that didn't have GT5 as an alleged system seller. The October to November increase was also lower than in 2009 (280k -> 389.8k). It was similar for Prologue which launched in April 2008 (March 2008 to April 2008: 257k -> 187.1k; although it at least did see a YOY increase of 105.1k).
So no, it's far more complex than that, there are many factors to monthly hardware sales, and not all people who would buy a system for a game would do that in the month of its release.