I don't think for a second that he thought to himself, "Let me go out on their floor for 2 hours and work up a sweat after the game, when I could be showering, just so that I can get under their skin." As the article mentioned, it was an action that "showed respect to the Heat". It was him saying, "Oh crap, these guys (as a team) really are that good - they are a serious threat to my goal of winning a 6th championship." He went out there and worked out for 2 hours to sharpen his skills, get in a few hundred more shots - not to play mind games. The writer of the article kind of posed it that way - like when he said that this was Kobe's way of saying, "Enjoy your evening on the town Miami, while I sit here and shoot jumpers in your arena." I don't think that was Kobe's motivation. His motivation was internal. It was about the Lakers getting better, not about showing up the Heat.
Sure, I guess if one was pressed to find a weakness in the game of one of the best players in the world, that would probably be the weakness you would point out first.