Could you explain this? I’m assuming you wash your hands after using either tp or a bidet.
I think he is talking about all the water that goes into making TP.
OK, I’m an idiot. 
Older landfills were not designed with much thought to the decomposition of their eventual contents. There are occasional stories of methane pockets moving underground, infiltrating cracked sewer lines, and causing toilets to explode when someone dropped a cigarette butt into it. Modern design and operation of landfills can include the addition of enough water to support decomposition to each day’s deliveries and piping to collect methane. Lack of water is the main reason for things in landfills being preserved instead of decomposing.
OK, but if you want things to not decompose, for the purpose of sequestering carbon, it’s really easy to make that happen.
Basically you don’t want an old unlined landfill to be wet because that garbage water will infiltrate the groundwater below. That means keeping it dry, which prevents decomposition, but you also have to compact it as much as possible to not only prevent rain infiltration, but because if it doesn’t decompose, it takes up a lot more space.
Newer lined landfills have a huge and very thick rubber or plastic membrane underneath to prevent water from getting out. That’s an expensive initial investment (it has to be dug out to create a bowl first), so allowing the garbage to decompose will reduce its volume over time, meaning less need for compaction or additional acreage.
Japanese tend to not use toilet paper at all. Now it doesn’t sound so weird to me=)
Except that the problem is that decomposition doesn’t decrease the volume of garbage. It vastly increases it, by multiple orders of magnitude. It’s just that the high-volume form it converts to is invisible and spreads out rapidly, and we humans have an unfortunate tendency to ignore things that are spread out or that we can’t see.