Games usually take about 3 hours anyway, so implementing a "tournament mode" that doesn't allow saving wouldn't be that big of a deal for players who wanted to compete. However, it is one more thing that Kornel would have to implement if he wanted a web scoreboard that wasn't easily cheatable.
Even with replays, people to watch replays, a serperate mode, and whatever else have you there would still be more ways to cheat the system if people really wanted to (And I'm sure someone would just to feel special). The only way to have a truly corruption-free high score list would be to host an entire DoomRL online server, and I doubt Kornel wishes to spend the time or resources for that, as cool as it would be.