I guess weapon needs to be explicitly defined in the rules. I would not consider a wedge a weapon because it does not cause damage to the opponent and its intention is not to cause damage but just to break their traction or get under them. Similarly if you were pushing the opponent around with the flat side of your chassis, or driving on top of them with your wheels, these things (your chassis and wheels) would not be called weapons, even though they are interacting with the opponent.
Similar inactive devices like a spike would be classified as a weapon because of its purpose to cause damage by stabbing into their armor or wheels. But I still would not consider a greased up spike a "liquid weapon". And I could not see how this grease could "spill out" or cause "significant cleanup." Wiping up some grease is way less clean up than they already have to do for the spinning weapons such as Last Rites and Touro Maximus (bolting beams back down, cleaning up shattered glass from smashed ceiling lights, mopping up tons of shredded tires and armor that blankets the floor...). These right here are weapons that require significant cleanup AND damage the arena.
I think the only thing that really counts as a liquid weapon is if you are trying to spray or pour liquid on the other teams bot in order to disable or damage it. Like if they didnt have some rule, someone might try to pour acid on the opponent, or liquid metal, or gasoline... all of which would be a huge mess. I think thats the only intention of the liquid weapons rule, to outlaw this type of thing.