I believe it. Even so, my experience with tires is that certain rim+tire combinations don't work. Of course that won't help OP; but it explains why some of us have more trouble with these tires than others. I have a Marathon Plus on the rear wheel (Sun CR-18 700c) of my tandem, and it's not particularly difficult to manage it. I have a regular Marathon on the rear wheel of my folding bike (Alex 16 x 1.5) and it's so loose I don't even need levers (but seating it can be tricky).
My experience differs from some as I have worked as a mechanic and have my own little shop... I change and install a great number of tyres from many makers and have sold a lot of Schwalbes of many flavours.
I always tell people that if they are going to be self installing the Marathons that they can be problematic and most folks will opt to have me do it... it's a matter of using perfect technique and tools and saying the right prayers to the bike gods.
The smaller sizes seem to be more problematic... the 20 by 1.5's on my folder were a pita to install initially while the 26 by 1.5's on my longbike and touring bike were not nearly as problematic.
I can hand set just about any tyre but if there is a consistent exception it is Marathons.