Hi jk82012,
They use an unusual design theory that they have had good success with even though it is quite different from the norm used by most manufacturers. Foam density and foam firmness/softness are not closely connected and any density of foam can be made in many different firmness levels. Density is the single biggest factory in quality and durability of polyfoam (although there are others as well) while firmness/softness levels are part of preference and have much less to do with quality and durability (although they do play a lesser role than density).
They don’t describe the topper because each customer can choose the firmness level that they prefer. They will call you to find out what this is before shipping the mattress.
You are also correct that while this is high quality base foam (and higher than most) it is not technically HR polyfoam. There is more about this in post #5 here. In essence … the term HR is widely used as a descriptive term (meaning this foam has higher resilience than most) and sometimes as a legal or technical term (where it meets the actual specifications for HR polyfoam and can be included on the law tag). The key is to know the density of the foam so that you can make more meaningful comparisons. I wish that it was used correctly in every case but unfortunately it isn’t and “competitive pressures” mean that in some cases, someone who uses terms like these in a more correct and technical sense can easily lose business to other outlets which are using lower quality foam but also calling them HR. In some extreme cases … I have even seen 1.2 lb polyfoam labelled as HR which of course is ridiculous.
Thinner layers of foam will get firmer faster than thicker layers because they compress to a greater percentage of their thickness more easily. This is why layer thickness and layer firmness are both important and interacting parts of mattress design and construction. Much of how and why a specific mattress feels a certain way to a specific individual can be counterintuitive which is why in some cases overanalysis can be more confusing than helpful.
It is actually quite rare to see true HR polyfoam used in the support layers of a mattress and 2.35 lbs is actually a higher density than you will normally see … even in more expensive mattresses. Compared to most (but not all of course) the use of 2.35 lb polyfoam is actually a bonus rather than a “cost saving”. The most common base foam densities are in the range of 1.8 - 2.0 lbs.
You are also very correct that thicker layers of latex also come at a price (because the material cost is higher) … but it can also increase “value” 
Phoenix