When they started with episode 3 they’ve already worked on Half Life 2 for more than 8 years. Most good ideas had already been explored, and they struggled to come up with new ones.
At that point it’s easier to start with something fresh where they’re not confined with the expectations of what a Half Life 2 should be.
It’s a weird argument for them to make: We are too exhausted to make a short game so instead made an entirely new full game.
I assume that’s how creativity works? Making new different things gets you more ideas than doing similar thing over and over.
When they started with episode 3 they’ve already worked on Half Life 2 for more than 8 years. Most good ideas had already been explored, and they struggled to come up with new ones.
At that point it’s easier to start with something fresh where they’re not confined with the expectations of what a Half Life 2 should be.