None of his Pokemon have enough SpA to deter Excadrill, I'll grant you Carracosta is annoying with Sturdy but it normally Shell Smashes and still won't get the jump, that's 32 Speed for you. Wailord and Jellicent get 2HKO'd at worst with EQ. Excadrill's not the best thing to use against Marlon but the fact that it CAN actually beat a leader in a type it's weak to is at once a testament to Excadrill's brilliance and Marlon's shittiness.
With Sandslash I'm on the fence about its extra Defence (no pun intended), on one hand it gives it an edge over Excadrill against Fighting and Ground-types but on the other Excadrill's Steel typing means it resists loads of stuff and takes less than Sandslash would anyway. Also Excadrill is much faster, meaning it might never have to take the hits that Sandslash will.