It's true that we might be splitting hairs here. Better is the enemy of good enough. If manufacturers anticipate that their products will be consumed relatively quickly, the difference in permeability between HDPE and PETE is probably not as important as other production factors.
I'm still interested in what you folks have to say about light sensitivity of developers, though. All of the liquid concentrate developers I've used came in non-brown bottles. I think Kodak did use silver plastic bottles years ago, but now they seem to use ordinary (natural) translucent HDPE. So, I'm still curious as to whether there is really any substance behind the recommendation to keep developers in brown bottles.