Yeah, retrofitting stats has been around the game longer than chewing tobacco and hot feet. You can get save stats from the 1910s and 1920s despite the save not bring a stat until 1969. If you think about it, this wasn’t easy to do. I mean, you needed to review every game for what the score and base runners were when the pitcher entered the game, make sure he didn’t lose the lead, etc. For every pitcher/every team.
As for WAR, there are no defensive metrics on Fangraphs before 2003 (I think?), but there are WAR numbers. The offense is based on Runs Created, which is a calculated stat with many formulas. The most common is RC = (H+BB+HBP)*TB/PA or something like that. No idea how they got defensive runs or base running runs, possibly just simple formulae using TC, A, PO, E, etc for defense. No idea on baserunning.