It was originally designed as inner duct for pulling fiber into existing banks of 4" conduits. Three or four of these would be pulled in to create separate compartments. The walls were thinner since it didn't really need much in the way of crush resistance.
Inner duct eventually migrated indoors as IT guys liked to use it for added protection for fiber cable. It's not necessary, but some people just prefer it. Since it was IT guys making these decisions, the popularity of using inner duct took off dramatically and Carlon was one of the larger manufacturers of it. They came out with plenum versions, smooth or corrogated, multiple colors, etc.
Eventually, this popularity extended into the low-voltage industry everywhere. Speakers, CATV, phone, you name it. Carlon seized this opportunity by developing a completely separate line of products for this use.
So, in reality, this was never really meant to be directly compatible with ENT at all. It was a separate product line that had much less stringent requirements.