You need to understand the chemistry of modified verses unmodified...a modified mortar has components that must dry to achieve the system's full strength. It must outgas water vapor for this to occur. Essentially, a modified is a layer of a laytex type material that encompasses a matrix of cement. The laytex provides a protective barrier around the cement and is stickier. Until the laytex dries, it is easy to tear the bond. Think rubber band verses a concrete block.
Ditra is essentially waterproof. Porcelain tile is essentially waterproof. The drying can only occur via the grout line gap, and if you decide to fill it with grout before that happens, it can literally take months to achieve its strength (concrete based products strength is based on curing for 28-days - an unmodified achieves more sooner when placed between two waterproof surfaces like Ditra and porcelain). While, eventually, this will happen, if you walk on or move heavy object over the floor, you may crack a tile or break the bond. Because the manufacturer has no control over that, and few installations would restrict access for that long, they tell you to use a (good) unmodified. They will give you an approval to use a rapid set modified, but that is not something a typical DIY'er should ever consider. A rapid set modified will achieve enough strength to prevent most disasters, but will still take forever to achieve the stated full strength. IOW, a good unmodified may provide equal strength, but not quite as fast. The rapid set would allow a pro to potentially set and grout in the same day.
An unmodified thinset only needs to cure...if there were excess moisture (within reason), it would not affect the curing time or the strength (too much moisture, and you will compromise the strength, though). This is predictable. So, while a modified may have a higher ultimate shear strength, that difference would be lost if you didn't allow things to cure AND dry properly, and it means an extended 'keep off' time.
A good unmodified thinset (not the entry level, cheap price leader which is mostly sand!) will typically have a shear strength of 300+ pounds/sqin. A good modified may increase that by a third. But, take a typical 12" sq tile 12*12*300=43,200 pounds! Are you really expecting to need more than that? The Ditra would debond long, long before the tile would break off the mortar.
Compressive strength is in the same ballpark, and if you had a point load big enough to be an issue, you'd crack the tile first anyway with either.
If you want to fill the waffles first and can let it sit for a couple of days, you could use modified for that. But, I'd still use unmodified to set the tiles on it. I've dumped a clump of modified out on newspaper and let it sit for a few days prior to throwing it in the trash...in the center, it was still soft and springy. That doesn't happen with an unmodified as it will cure anaerobically.
Any manufacturer (with any kind of integrity) spends a lot of time prior to making recommendations on how to use their product...do you really feel you have enough info to second guess them?