Nope. Sheldon was wrong about this. Suntour Accushift 7s has 5.0mm spacing for the first 3 cogs, and 4.8mm spacing for the last 4.
In practical terms, this means a Suntour shifter and derailleur will not index perfectly with a Shimano cassette or freewheel. Whether it's "acceptable" or not depends on your tolerance for precision.
The reverse is less true: because the Shimano derailleur has a floating upper jockey wheel, it can accommodate the small difference better, and will index more acceptably on a Suntour freewheel or cassette.

As far as the OP sticking with GPX and Suntour 7s, I say do it man. I've been riding Suntour 7s for almost 30 years. Suntour's indexing uses overshift built into the lever, so cable drag can result in the derailleur not being able to pull the cable back through to finish the shift. Thus, it works very well on road bikes with downtube shifters because there is so little cable housing. It works somewhat less reliably on bikes with handlebar shifters and longer housing runs.

Suntour did make a 7s freehub and cassette but it was IME not as good as Shimano's until the Microdrive version. Much harder to find Suntour 7s cassettes than freewheels anyway.

Regarding Accushift vs. not: This is a little tricky. Suntour made New Winner freewheels in 7s that were pre-index (and even some Winner/New Winner that are not Accushift.) They are slightly narrower overall than index-compatible freewheels. Stick with Accushift or AP compatible Alpha, Winner or Winner Pro freewheels for best index results. IME the majority of Winner/Winner Pro are indexing compatible.