The Pontiac Tempest never had Grand in front of it. Now they may have considered the GTO as the Grand Tempest, but there was never a Tempest that was referred to as a Grand Tempest before the GTO. At least as far as I could find. The GTO did look like the Tempest. There was the Grand Am, but that was after the name Tempest was retired. There was also The Grand Prix, the Grand Safari (station wagon), and the Grand Ville
The key word there was “internally.” That seems to indicate that internally Pontiac was using “Grand Tempest Option” as the secret code name for the project. I hadn’t heard that before, but it makes sense. Project teams frequently code-name special projects before the marketing team dreams up the product name that the public will see.