since he still controls the money, and can fire the CEO, he could in theory veto anything.
in practice it's a very different matter. nobody is going to remain as CEO if they have the company owner/board of directors meddling in the running of the company.
this does get 'interesting' when a person is both the owner, and holds a position in the company other than CEO, but such things get worked out.
so this could mean absolutly nothing if shuttleworth wants it to mean nothing, and the new CEO is willing to go along with him, or it could mean exactly what it claims to mean, namely that Mark is stepping aside to put someone in the CEO position that he thinks will be better for the company than he would be (and it doesn't matter if that's because they will do a better job as CEO, or just free him up to work on other things)