LTP isn’t necessarily what you can ride at for hours… it’s actually a little more complicated than that. Essentially it’s what we expect your Threshold Power decays down to after hours and hours and hours of riding. IOW, riding at LTP after several hours will feel similar to riding at threshold when you’re fresh.
HIE represents your ability to generate power above threshold (anaerobically through the use of carbohydrates). As your body becomes more adapted to producing power anaerobically, you become less efficient aerobically - thus LTP generally decreases as HIE increases (given a constant Threshold Power). This also explains why professional sprinters aren’t winning Grand Tours (they lack the aerobic engines - think LTP - of the GC riders), and why GC Contenders aren’t able to hang with the sprinters in pack finishes (they lack the anaerobic capacity - think PP/HIE - of the power sprinters).