plants also consume nutrients in the water column that would otherwise raise the PH (not just CO2)
would have me thinking, the cap lowers the rate nutrients are released into the water column till it's slow enough the plants can keep the levels in the water column down
I have heard peat moss (sphagnum peat moss) when alive, can hold up to 40% of it's mass as water to deal with long periods without rain (or other sources of water to hydrate it). as this turns into a large store of water, the moss's natural defense to keep this water free from an infection is to keep this water acidic. I THINK this is where adding peat moss to substrates is done in an attempt to keep the PH lower.
i would think, (from your tests) this is more of an old-wives-tale than reality, ... (but honestly i don't know)