The Leica mono records the image on an individual pixel-per-pixel basis, the Fuji requires at least three pixels (R,G & under the X-Trans array filter to make up one colour site, which is then desaturated for the digital version of B&W. The Leica is thus already well ahead in the potential resolution stakes.
Things are still as they were in film days - B&W film had one emulsion, whereas colour film three emulsions stacked and separated by colour filter layers, and looking or printing through these layers combined to form a colour image. As a consequence colour film was never as sharp or as high in resolution as was B&W film of the same ISO.
Anyone who tried to print B&W from colour negatives will know the quality problems encountered with that workaround - on B&W paper (blue sensitive emulsion) the cyan layer was disproportionately represented as the paper was almost entirely non-responsive to the magenta and yellow layers, which led to awful skin tones and freckles where none were obvious beforehand. Printing on Kodak Panalure panchromatic B&W paper was a solution to a degree, except that it had to be used in total darkness and the images, formed from the colour dye grains that make up a colour neg's image, was never as crisp as it would have been from a proper silver B&W neg (as opposed to "chromogenic" B&W negs - e.g. Ilford XP-1 which were formed from a single-layer colour film emulsion reduced to dye in the colour processing chemistry used to process them).
Given this, we would carry two or more camera bodies loaded with the different films, or for medium or large format carry the different films loaded into separate film backs or dark slides to be used as needed on the one camera (a lot handier if the camera is set up for a shot on a stand or tripod, for instance).
Leica is simply returning to this philosophy, that a bespoke camera (or film) for the required task be used, not one camera (or film) with the resulting image doctored afterwards to imitate that which it's not.
Digital B&W has always been crappy compared with film B&W, Leica are just pointing out why and what's needed to rectify that. Whether they succeeded or not will be revealed by people doing such comparos as this. The other trick would be to convince enough people beyond the usual dedicated badge purchasers that the concept is worth spending the money on to make its continued existence worthwhile for Leica itself.
Much and all as I love the X-Pro1 for its almost-film-like B&W renditions, it does get awfully grainy awfully fast in B&W (for the reasons already noted), and it's almost the case that I'm contemplating the Mono with a 35mm lens for any purist B&W work I might want to do. Given years spent in a B&W darkroom, the idea does make sense, despite my reservations as to the rather dated operational aspects of the camera itself. The decision is really down to whether I can get the former to override the latter, or whether I just keep doing as I have been doing, and using my RZ 67 & ToyoView outfits with proper B&W film for any specific B&W only task (though they really are heavy and bulky to the point of hate).