It hasn't been established at all that the Atari 7800 wasn't good at scrolling.
All we've had is a handful of people stating that scrolling titles were something the console wasn't designed to do well. And their entire basis for that statement is because platformers were nearly absent on the platform due to Atari not ever bringing the console much past early 1980s single screen arcade titles.
The fact that a genre doesn't really exists doesn't make their statements on the 7800's hardware accurate. There are plenty of 7800 releases with scrolling that work very well, such as the port of Xevious. Even the Atari 2600 could handle scrolling very well, as dozens of releases like River Raid show.
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