There are multiplayer runs listed although the rule set says singleplayer? Am I able to submit runs including multiple players?

Not sure, the category names are also outdated. Very hard to navigate the ark leaderboards sadly.

the ark leaderboards have been basically offline for the last year or two. the mods have all given up and no one else really has taken the opportunity to fix ark speedrunning. the original mods were thrown out due some supposed nefarious activities and the new mods have lost interest in ark completely after screwing up the entire board, so the leaderboards have been non-existent. if you want to do an ark speedrun, make up your own rules and stick by them or go search for an old speedrun on what those settings were