Hi JB Bedman.
Welcome to our Mattress Forum! 
I am sorry to hear that in addition to your Tempur-Pedic mattress sagging your warranty claim has also been dismissed without deeper examination. As you may be aware warranty claims denials are fairly common.
The answer you received reflects a boilerplate type of answer and I doubt that Tempur’s Zendesk support is using a knowledgeable representative who can understand the technicalities behind the mattress foundation support. You’ve most likely interacted with level 1 & 2 customer support reps with no qualifications to comprehend abut the bed support details you provided. Before you take a more aggressive step, I would reply and ask to escalate your issue to a manager who has the ability to understand those details as your foundation does indeed exceed the recommendations that Tempur-Pedic requires for mattress support. In addition to emails, I would also ask to talk to the manager to ensure they correctly understand your notes.
I’d also keep in mind that even you succeed in them recognizing that the support under your mattress is better than what they require … generally major brand mattresses that use the lower quality/density foams (especially in their comfort layers) will end up softening and breaking down quite quickly. However, in many cases, the visible impressions created by foam deterioration aren’t deep enough to qualify for a warranty claim even though foam softening (virtual impressions) can result in the loss of comfort and support and the need to replace the mattress. In these cases (which are very common) there would be no warranty coverage even though the mattress may need to be replaced. I would measure to see if the visible sag is deep enough to qualify for a warranty claim (You do not state what impression depth you measured but their Warranty covers only visible indentations greater than 0.75 of an inch). Mattress warranties only cover defects not the loss of comfort and support that comes from foam softening so they have very little to do with the durability and useful life of a mattress. There is more about mattress warranties in post #174 here.
You certainly have a large audience that would be interested in the outcome of your Tempur-Pedic claim.
Phoenix