DOS is old code, so the humor comes from the paradoxial juxtaposition of touting the use of old code while simultaneously saying, "friends don't let friends run old code."

This superposition of statements is set against the backdrop of the tech idiom, "friends don't let friends" followed by some activity. [1]

This tech idiom is based on a decades-old advertising slogan, "Friends Don't Let Friends Drive Drunk," by MADD, Mother's Against Drunk Drivers. [2]

[1] https://www.google.com/search?q=%22friends+don%27t+let+frien...

[2] https://www.youtube.com/watch?v=qVuu8G8ww6s