Personally, I believe this, along with every other Sly Cooper game in the series should be ALSO considered a platformer. If you apply the site's definition:
Platformers feature obstacle course like levels in which the player must navigate from the start to the end, with physics-enabled traversal between platforms or over obstacles. Platformers require the environment itself to be a primary antagonist. Accuracy-based jumping challenges and endless runners are two common examples of Platformer. In all cases, correct timing is necessary (either to avoid death or serious set-back) to advance.
THIS series in itself was the definition of the 3rd name brand platformer of the PS2 era...along with Jak & Daxter and Ratchet & Clank...and was the next generation of the PlayStation Platformers. Puzzle solving, Stealth and navigating the level itself were the major challenges of the series and these ALL are requirements of a platformer game.