I don't know what you mean by this. For example, if something is hard-panned left, any headphones including MDR-7506 is going to put that in your left ear. In contrast, listening over speaks won't put it nearly as hard left. Listen to You and Your Friend (Dire Straits), which has hard pans. It's incredible on speakers, and annoying on headphones, due to sparse instrumentation with the hard pans. So I'd say that speakers are "more mono" than headphones. I don't see how you could say the Sonys are "nearly mono". If they were dull, maybe I'd think you meant that they don't locate transients well, but that's not the case, they are mid-bright.
I had to pull them out for a listen, because I haven't used them in a very long time, but I don't get that sense, listening to a few reference mixes.
Anyway, I doubt that was Scheps' motivation. He spent a long time mixing on analog hardware, and most of his stuff was center or hard panned left or right, because he didn't want to switch the panning amps in, due to the noise they added. I think he just gets the mix he wants to get, and doesn't want to change it to be more mono friendly.