Well taping the joints with Kerdi Keba & Coll is waterproof mate isn't it. In fact it'll be the only part of the UFH protected from moisture, the rest is just covered with tile adhesive cos unless you tank the Ditra Heat, it'll only stop water going thro the matting, and Ditra don't advocate latexing over it either.
Not that latex is waterproof, but if you tanked it, it would be.
And don't Impey suggest tanking before UFH, so I can't see difference really mate.
Or am I missing something?
And if you're using it for moisture equalisation, you don't need to tape the joints anyway, unless it's moisture sensitive stone.