I don't think there's any mystery to the showroom discount and I don't think there was anything misleading in that original email either, but it could perhaps be misunderstood by somebody not familiar with the inventory purchase process.
I just bought an inventory MX P100D with a substantial "showroom discount". Depending on which piece of documentation I look at, it's referred to as "showroom discount", "showroom adjustment" or "price adjustment".
My understanding is that this discount, applicable to inventory only models, is based on the original configuration price, discounted by 1% per month since manufacture and by $1 per mile. For example, my MX P100D was originally stickered at $147,250. It was manufactured in Nov 2016 and I purchased it on July 31, 2017, 9 months old counting the month of manufacture. It had 7,448 miles at the time the 1st quote was provided. My showroom adjustment was $20,600. Based on a 9% discount and $7,448 for mileage, it comes out to a $20,700 discount. By the time my trade was appraised, financing approved and I decided to put a deposit down, the final quote reflected a $20,900 showroom adjustment and 7,525 miles. I bought this just before the recent changes to PUP, premium interiors and the base price adjustments, so I don't know exactly how those are calculated or applied to the discount.
What I don't understand, is how Tesla determines which inventory/demo models show on the Tesla website new inventory search. The teslainventory.com site shows 32 Model X P100D or 100D models and tesla.com shows no inventory models w/ 100kWh batteries. 12 of those are new, with under 50 miles, the rest look like demos. The car I purchased actually didn't show up on either site, my OA found it for me.