Is it normal that only a little bit of water could have gotten out of the bladder and into the tank? Because it still sounds hollow when I knock on it and no water came out of the valve on top.
A diaphragm is a flexible membrane which fully separates the air and water chambers.
The air chamber occupies ~3/4 of a pressure tank's total capacity. I don't recall your tank size being specified, but for example, if your tank size is 40 gallons, it will have an approx. 10 gallon drawdown capacity and ~30 gallons space for air. This is when the tank is filled to the pressure switch cut-off pressure. When the water pressure is lower, the air chamber will be expanded, reducing the space of the water chamber.
You estimated your tank, even after being drained, to weigh 80-100 lbs whereas the manufacturer specifications indicate it should weigh 40 lbs when empty.
Water weighs 8.33 lbs per gallon so if your tank weighs even only 60 lbs after draining (ie: 20 lb difference), that would suggest there is 2.4 gallons remaining trapped in the air chamber. 80 lbs = 4.8 gallons. 100 lbs = 7.2 gallons.
Whether 2.4, 4.8 or 7.2 gallons, the space within the air chamber will remain substantially vacant of water, so knocking on the side will likely continue to sound hollow. Because the water in the air chamber is sitting directly on the diaphragm which separates the air and water chambers, with no pressure below the diaphragm to deflect it upward, the water in the air chamber will not be pushed out from the Schrader valve located at the top of the tank.
Seems like most of the time people's tanks are completely waterlogged when the diaphragm has any sort of leak.
For the air chamber to be completely water logged, would mean the air chamber to be completely vacant of air. For this to occur, would require all air to leak out from the air chamber, such as resulting from a pinhole leak through the top of the tank's shell or through the Schrader valve.