Actually, yes, a bearing does need to be near a heat source to experience heat. Do wheel bearings in dirt bikes get hot? Not really, compared to streetbike wheel bearings or trailer bearings or the like. They turn at much lower speeds than those, due to both the larger diameter of the wheels and also the lower overall speeds of the vehicles. They also don't get cooked by braking heat. Besides, he said nothing of wheel bearings in his post.
Since he's working on a YZ490, the YZF's penchant for melting the head bearing grease is also not relevant. The Bel Ray I've used on my CR has worked just fine in every application he mentioned, so I see no need for high-temp grease. I also don't know what they put in high-temp grease to make it more temperature resistant, so there is no guarantee that those same additives don't make it less effective at lower temperatures and therefore an inferior product in this application. There are no free lunches in life.
As for "considering the cost of grease," I think you have it backwards. Grease is relatively cheap, so buying two different kinds isn't a significant expense. Then you have the option of using the best product for the job at hand, instead of a one-size-fits-all solution. I've probably got a half-dozen different kinds of grease in my shop, all for different applications. That same tub of grease will also outlast a dozen motorcycles, so the cost isn't even worth considering.