My interpretation is he's being a little sloppy with his wording.
If he has a full bowl and no water going from the tank to bowl (the tank being the only water source for the bowl), and he "hears leaking" but the water level in the bowl doesn't drop, then he isn't hearing a toilet leak. He's hearing something else. Is there a heel inlet to his closet bend that is connecting water from the shower or sink drain? Is there water dripping in the stack which is located 3 feet down his closet bend? Is there cheapo piping in the area through which he is hearing "running" water from another part of the house? I don't know. What I do know is that if there's no water going into the bowl, and the bowl level doesn't drop, then there's no leak from the bowl.
More often than not, what the guy would have is a slow leak from the tank to the bowl that he isn't perceiving, maybe because it's entering the bowl through the siphon jet, and that slow leak of water into the bowl is causing a little water to dribble over the weir. This of course would be accompanied by an occasional activation of the fill valve to refill the tank. He says he fixed the possibility of this by replacing the flush valve and flapper. Fine. I take his word for it.
The other thing it could be is simply the bowl water level settling. Most older toilets overfill after a flush and then settle slowly. Water drips over the weir for a little while. Maybe the floor gives a little when he walks into the room and a little water flows over the weir.
In short, I don't know what he's hearing. If there is no loss of water height in the bowl with no water going into it, however, what he is not experiencing is a leak through a crack in the bowl. (Now if the bowl fills to its normal level and then drifts down a smidge to the level of the crack, well, then...YEAH. But otherwise no.)