Hi MelissaJane,
A high BMI presents special challenges and generally requires firmer materials in the support layers especially. This could be firmer latex or innersprings (the type of support component would be a personal preference and in the right design either could be suitable) or even a zoned construction. I believe that the reason for their suggestion is that they don’t carry the firmest latex layers that wouldn’t be necessary for most people. The same overall guidelines apply with higher weights though that PPP (Posture and alignment, Pressure relief, and Personal preferences) along with using high quality durable materials that will maintain their feel and performance for longer periods of time are the way to make the best choices. Heavier people in general will need firmer and thicker comfort layers and firmer support layers than those who are lighter and because no materials will last as long with much higher weights the quality and durability of the materials and components is even more important than normal. I wouldn’t “rule out” any types of mattress and base your choices on your own personal testing.
As with any mattress purchase … the best way to know which mattress is most suitable is with personal testing and the guidance of a manufacturer or retailer that has good experience with higher weights because there are too many variables, unknowns, and personal preferences involved to use “theory at a distance” as a way to make a mattress choice (see mattress firmness/comfort levels in post #2 here).
There are also other options in the same area that are within reasonable driving distance that are listed in post #2 here and would be well worth talking to.
Phoenix