That timeline doesn't seem quite right. Unless I am mistaken DOOM was originally released in December of 1993 and the source was released December 1997. That's

That timeline doesn't seem quite right. Unless I am mistaken DOOM was originally released in December of 1993 and the source was released December 1997. That's only 4 years, however Quake was released in June of 1996, which is close enough to 2 years. It is also worth noting that commercial releases of DOOM continued through 1997 on various home consoles even if you ignore the various resurrections it has had for the likes of the GBA or Switch. Edit: I think I get your timeline, you're referring to the re-release of the DOOM source under GPL in October of 1999.