Cycling is environmentally friendly, but let's not equate world championship to cycling as transport. The event itself must have a lot carbon footprint. Still, weird choice of event to protest, but I can see them doing what they can to get the publicity they need.
I hear this a lot, that "it's not worth doing anything because of <insert reason here>", or "well, the solution is not perfect so why bother?".
Any improvement is better than nothing. If no one ever changes, nothing will ever change. Just because someone else is being worse, it doesn't excuse you from being bad.
So yes, it's a drop in the ocean, but at least it's something. Little drops add up, and they make it harder for the real polluters to justify/excuse their actions.