If you ask Schluter, and this has been true for many years, IF you use a rapid setting modified mortar on their products, (i.e., one that sets properly in the presence of extra moisture) they will approve its use. That they don't advertise this has a good basis...their extensive testing for over 20-years says that stuff bonds to their membranes just fine without a modified. Should you disagree, you've always had the option of getting permission from them. FWIW, there are many more products that will work other than Ardex's. But, in the vast majority of situations, you don't really need them.
The second part of this is, and this supports CX's stand, is that if you use Ardex products with Schluter membranes, your warranty is from Ardex, not Schluter UNLESS you got prior approval for their use. So, if you don't want finger pointing, and want to go to one source, follow the proper procedure. Schluter does NOT publish for the NA market nor recommend the use of a modified thinset with their products (to set the tile - on something like Ditra, to bond it to say plywood, it is REQUIRED, but not on top to hold the tile). So, there is no Schluter published warranty for NA to set tile on their membranes with a modified...only permission on a case-by-case basis (at least that I have ever found).
When Schluter brought their membranes to market in Europe, they tried to get each individual country to approve their use (as it is used here)...but, the politics made that an impossible task. Since they wanted the same procedure for the full Eurozone, they relented and worked out a method that would work. When they brought their products to the USA, after review of the science and technology plus the test results, they got approval in all 50-states, and in Canada to install it as it was designed. They just had to convince one certification group for the entire country...politics and trade group pressure didn't play a part in it.
It works, it passes all industry reliability tests. Tile doesn't fall off of it if you put them on as directed.