My own impression has been that he does not really care one way or the other about tivoization. If that clause had been present in the GPL since the beginning, it would not have stopped him from using it.

However, he sees things like adding it as being a pretty significant change to the GPL, and does not like the fundamentally changing the deal. If GPL v3 consisted only of wording clarifications to ensure it worked properly in more regions and perhaps to allow apache license compatibility, he would presumably have had no objections, but of course, would be unlikely to have been able to relicense the kernel, given the kernel's numerous contributors and lack of "or later version" clause.