No, what he's talking about is the fact that Pac cabinets use a standard horizontal shelf mount monitor, but turned sideways (vertical). This is different than some later vertical games that use a _vertical_ mount monitor, mounted "normally".
The ONLY difference between a vertical mount monitor and a horizontal mount monitor is the metal frame itself. Most monitors were designed with a "horizontal" mount frame that sits conveiently on a shelf - but some were designed with a "vertical" frame so they too, could mount conveniently on a shelf, without having to mount the horizontal frame sideways. But electrically, the two types of monitors are identical. They're just fitted into different metal frames.
It's really best to stick with the frame type that the cabinet's mounting hardware was designed to use, if possible, unless you can come up with a simple way to do it. The last time I had to do something like this was when I needed to mount a vertrical style monitor into a Tron (which uses a horizontal shelf type monitor, mounted sideways). I just swapped the monitor frames, mounting the picture tube and chassis from the vertical monitor into the horizontal frame of the old monitor. Had to get creative with the chassis mounting, as I was putting an Electrohome chassis into a WG frame, but it went together well and worked perfectly.
-Ian