I think that there are enough of internet warriors that someone like Dawkins doesn't have to involve in those arguments.

Also making someone change his mind on the internet most of the times is close to impossible, so it's usually pointless.

It's not only that. People making searches can end up finding those tweets. If the only thing that they see is the wrong statement of trolls, they can think that those statements are true. On internet, repetition is assumed as real so if only the wrong things are repeated, then people will accept them as truth.