Yes, incorrect. Whatever your source was for that I'd stop using it.

The tstat probe goes outside the tank so it can not be moved or urinated on. If either of those things happens the tstat will think it needs to ramp up the power to hit the desired temp. This will then make the uth run away hot with a good chance of burning and even killing it.

The substrate over the uth should be no more than 1/4 inch thick.

The thermometer probe can go on top of the substrate or under. BUT, no matter where you put it you need to know the temp of the actual tank floor. A IR temp gun is the best device to figure that out. You always want to know the temp your snake could reach. They will and do move substrate.