I think you're technically correct, but that's not how these games used those graphics modes.
A game with "CGA graphics", used the 320x200 mode, which had one of two fixed four-colour palettes, red/green/yellow/black or cyan/magenta/white/black. It was godawful.
A game with "EGA graphics" used the 320x200 mode, and the 16 colour fixed palette. This mode overlapped perfectly with graphics modes on the Atari ST and the Amiga 500, which allowed game developers to make games that looked exactly the same on all three.
So all the classic LucasArts games, from Maniac Mansion to Monkey Island simply ran on a port of SCUMM on each platform, and re-used the graphics assets straight up.
Sierra did the exact same thing in this era, they had their own game engine, and games like Police Quest II, Quest for Glory and King's Quest IV looked exactly the same on all three platforms.
