They took the completed game and worked backwards to the underlying code that makes all of it work. They confirm it's an exact match by using the code they worked backwards towards to make the game again and confirm they have an exact match to the orginal game file(s).
Once this is done, it is possible to change literally anything about the game, as you have access to the code that makes it work. One of the most popular things to do is to replace the parts of the code that are specific to the console it was originally made for with code that can run on PC. This results in an accurate PC version of the original game. After that, other coders usually dig into the PC "version" to add all sorts of easy modding support.
This has already been done with Super Mario 64, Ocarina of Time, and Star Fox 64, and they all have easily moddable PC versions now. This has resulted in things like Render96 and Ray Tracing support for SM64, a bunch of bug fixes and an insanely deep randomizer for OoT, and discovery of cut levels and entire game modes partially left in the game code that had only been mentioned in interviews in Star Fox 64.
It's also worth noting that similar projects exist for a whole bunch of other games. The Pokemon Emerald Reverse Engineering project has allowed all the absurd new romhacks with new features like Mariomon and Too Many Types 2, with Pokemon having 3 types and a total of like 70 types in game. Similar (but technically different under the hood) projects for Sonic Unleashed and the Jak series on PS2 have resulted in great moddable PC versions of those as well.
It's not hard to understand.
Dessalines and Nutomic, the two lead lemmy devs, think that Tianenmen Square didn't happen. They think the genocide currently going on in China of that particular group of muslims (Uighyur or something like that, I can't spell it right) is actually a social program for their own good. They think that Ukraine is legitimately filled with Nazis in their government and military and that Russia is rescuing the poor populace from their oppressors.
Effectively, China and Russia can do no wrong. Dessalines in particular regularly bans people from lemmy.ml who say otherwise.
The term "tankie" is in reference to the tanks China drove over people during the Tianenmen square massacre.
Edit for the sake of jackasses: Better explanation of the term "tankie" from another commenter here. And it's spelled Uyghur.
Happy now?