Ah, okay, it says:
These frames have a top bar which is the same thickness all along its length. This gives great strength but it does lift the frame up, reducing the top bee space, although this is countered by a corresponding increase in bottom space. They can be mixed with conventional frames with thin lugs but some beekeepers may prefer to keep them separate.
Then I misunderstood -- I thought they meant that the top bar is 10 mm thick, but what they mean is that the top bar is the standard 15 mm and that their lugs are also 15 mm thick.
This means that the frame sits 5 mm higher in the box, and if you have a box on top of that box where the frames are not 5 mm higher, then the frames are too close to each other, and the bees can glue them together.
Personally I would have thought that making the lugs the same thickness as the top bar is simply a cost-saving on their part, as it reduces the number of cuts you have to make while building the frame.
So, the warning about mixing is a warning that you might not get the beespace you're hoping to get.