That is interesting. But in the past, Azek was much more flexible than plywood, not even close.
I think it's possible to use, but will just need a few adjustments.
The transom may be a little rubbery, but if you are using a very small or electric motor, it might be okay. You might be able to use some stiffeners to help, if using a larger engine.
It sounds like you are planning to use 3/8" on the floor? That seems a bit thin. Are you going to texture, paint or carpet it?
Every time I've seen it used, it sags and sets in that sagged position over time unless there are multiple supports, so you may want to use lots of angle aluminum supports.
It can be like ice when it gets wet, so you may want to carpet or put textured paint over it.
Definitely post a thread on your build! If this is what you want, you should be able to be successful. And maybe they have improved the product to be a bit stiffer, which would better suit your application.