I think your conception of what someone in that role does is incorrect.
While it is different for every studio CEO, most effectively act as an Executive Producer on every project they green light. He was not “just selecting shows that other creators” made, he was reviewing the progress, giving notes, choosing (or at a minimum approving the choice of) the major talent positions (Director, writer, lead actors, etc.). For films (and a series like Game of Thrones is produced more like a film than a standard TV show), they likely even approve the script. The projects discussed were produced by/for HBO, not just purchased by them after they were finished (distribution deals like that happen, however, that is not what these projects were).
A more accurate description of his transition is from an executive responsible for all aspects of the business (content creation, acquisition, marketing, sales, etc.) to one just focused on content creation. He is not going to become a line producer, nor will he be focused on a single project. Just like when he was at HBO, he will have several Executive Producers working for him, each focused on specific projects.
He is likely to have much the same role as he did before related to content creation - choosing or approving the key talent, scripts and budgets for the projects he is developing - without the rest of the responsibility for the studio/network.
Just like in finance where “past performance is no guarantee of future results”, having a successful run (even in the same exact role - director, writer, etc.), does not mean that the run will continue. Robert Zemeckis made many successful movies and then made Polar Express, Beowulf and A Christmas Carol, all of which were terrible.
Like you, I hope he succeeds in his new role and produces great content for us to watch.
Yep... best of luck to him.
Let's hope he brings to Apple what he was able to cultivate at HBO.
The list of HBO hits was long!