I remember having Commander Keen on my 486 and it was the first game where I realized the levels were stored in plain ascii. Changing the ascii values would directly change the level... I lost so many hours of my life to that!
http://www.shikadi.net/moddingwiki/RLEW_compression
Looking at a hexdump of the Keen1 file matches those specs. Keen 4 uses another, more complicated kind of compression, too: http://www.shikadi.net/moddingwiki/Carmack_compression
They might be thinking of earlier Id software games. Some of those weren't compressed, and you could at least edit them with a hex editor pretty easily.
Similar simple edits often work on save file data from games of this era as well, and often don't need a hex editor etc.