I guess that's just a psychological thing. It's an expensive and much wanted piece of kit that they don't own and the keys have got screens on, both factors are probably going to want to make you be gentle with it.
I'm willing to bet that it's in the reviewer's head- the ML switches only take 1.6 oz (45 cN for the metrically inclined) to actuate, and the travel is only 3mm (compared to 1.8-2.3 oz and 4mm travel for the MX switches). I strongly doubt that the added inertia from the mass of the caps is really going to be noticeable. Now, the size, shape and spacing,
that I can see making a big difference. I seem to recall that the keys are spaced a fair bit wider, to make the screens usably sized, and the flat, smooth surface doesn't really look appealing, tactilely. Maybe the reviewer is mistaking strain from stretching for key actuation fatigue? I've seen a couple of other reviews that praise the action on it- one compared it to a Model M (though not actually by name- just "an old, clicky IBM keyboard")