Sadly I actually think it'll be on the contrary. This revelation is more like an advertisement for him now that it's out in the open so people know he can be bought and are more likely to seek out his promotion. People know a tweet from him is very valuable for their coin because it does have an impact on the price with all those idiots who are always looking to throw their money at something.
Also, buying tweets is nothing new. I read a while back that Christiano Ronaldo charges 250k a tweet to promote something. I've yet to hear about him promoting an ICO though but most of the time these prominent figures with large social media followings will promote anything as long as you pay them though something tells me Ronaldo will have lawyers that advise him on these things.