Hi craz4cakes.
While I wasn’t part of the phone conversation and I can’t interpret what you were told, your feeling is probably right that both retailers are trying to help you. I would point out that the overall the feel of a mattress is the result of all the layers and components working together, their arrangement, the ILD, thickness and many other factors including the cover, quilting and the actual encasement. Depending on the arrangement there can certainly be a difference between 10" and 13" thick mattress and it is better to think of the thickness as a byproduct of the actual design. The thickness of a mattress is just a side effect of the design and by itself isn’t particularly meaningful because whether a thicker or thinner mattress would be better or worse for any particular person will depend on the specifics of the materials (type, firmness, etc.) and on all the other layers in the mattress.
Thickness is only one of many specs that are used to make different mattresses that perform and feel differently and that makes a mattress suitable for one person and not another. There is more about the effect of thickness in post #14 here. Regardless of how thick or thin a mattress may be, the most important part of the “value” of a mattress is how suitable it is “as a whole” for your particular body type, sleeping positions, and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) regardless of how thick it may be. There is certainly no reason that at your weight you would “need” more thickness, but you may still “prefer” a mattress that is thicker or thinner depending on the design and layer arrangement and perhaps customization options as well.
In general, unless another manufacturer tried a product of another manufacturer they cannot speak about the feel of someone else’s product in detail, they can certainly speak about the componentry and durability of a product and the overall feel of the product. Even then this could be inaccurate as each person would have a subjective experience of the mattress, which could be different from someone else’s. Perhaps what was meant is that for their own particular design going with 13" inch vs 10" would not bring any benefit to you and it would be unnecessary particularly for your BMI and that you would not notice much of a difference (which is nice as are not trying to upsell the product).
As each manufacturer is carefully designing their products and in some cases improving on their designs based on customer feedback, they would usually base their recommendations on the “averages” of other customers that may be similar to you" This is a long way to say that both designs can work for you.
This is a good mindset to have a successful mattress purchase, but even then it is good to remember that an online purchase has its own “risks” and that sometimes a process of trial and error is unavoidable as nobody can feel what you feel on the mattress and predict with certainty how a particular product would work for you as you have unique sensitivities, needs, and preferences. You can read more about the pros and cons of a local purchase vs an online purchase in this topic and Post #2 here about dealing with experienced, educated and helpful manufacturers who have the knowledge to guide you through the process and find something that they honestly think will fit your specific sleeping positions, somatotype, BMI and individual needs.
I hope this addresses your concerns and helps you move forward in selecting the best option for you.
Phoenix