It is always difficult to try and second guess decisions which were made by other people behind closed doors. Particularly when you know the end outcome where the people who made the choices wouldn't have.

When Jobs wrote that memo, the iPhone wasn't remotely dominant. It had tiny market share on and Windows Mobile and Android were both very big threats to the iPhone. There could easily be an alternative history where Adobe wrote a really effective version of Flash and Apple was left out in the cold.

Fast forward 10 years and it all looks obvious, but I don't think it was a slam dunk back then.