Only one thing: EGA games back in the day didn't use to show us the mixed surfaces (the dotted ones) as they were blurred into another shade of the main color they consisted of. It was a brillant trick used by graphic programers back then to give the player the impression of more colors than there actually were there (and more than were technically possible with EGA).
Sorry but not true. I've researched this extensively when creating my authentic PC monitor emulation shaders for DOSBox Staging.
EGA monitors (and even CGA but to a lesser extent) were *tack sharp* as IBM PC monitors were designed for business applications first and foremost. The primary use case was staring at 80-column text and spreadsheets all day, and if you make 80-column text sharp at 640 pixels of horizontal resolution, pixels in 320 pixel wide modes will appear as quite distinct little blocks.
Consequently, checkerboard dither on EGA art was clearly visible, there was no blurring and blending going on. I can back this up with a collection of photos of real EGA monitors and you can check out my EGA shader examples on our website which are pretty good approximations of the EGA monitor look.
https://dosbox-staging.github.io/
If you think about it, 80-column text on 80s CRT TVs and lesser home computer monitors (which were basically small higher-quality TVs) was very hard to read and quite blurry. Such TVs blurred 320x200 checkerboard dither somewhat, especially via composite or RF input, but still not perfectly.
That's the reason why practically all home computers defaulted or only supported 40-column text. IBM's EGA monitors were digital (yeah, more than a decade before DVI!) and quite expensive, all in the interest of sharp 80-column text. An EGA monitor could cost more than your typical home computer.
I assure you that ScummVM feature is a fantasy setting and it's nothing how EGA games looked back in the day. Even my Philips CM8833-II (basically the same as the iconic Commodore 1084S) I used with my Amiga 500, which was less sharp than EGA monitors, displayed checkboard dither as distinct checkerboard patterns in Sierra ports. I did spend considerable time in Deluxe Paint pixeling, and on Amiga monitors you could indeed make dithering almost "melt away", but you had to use colours close to each other for that. That did not work with the very contrasty, by-the-numbers 16-colour CGA/low-res EGA palette.