Seat tube length definitely plays a part in bike sizing, but stack, reach, and effective top tube length are the main numbers I use when looking at bike sizing. For example, I’m 5’6” and I’d pick the frame size mainly by reach and effective top tube, and just check to make sure the stack isn’t ridiculous and the seat tube length is less than 420mm so I can fit a 150mm dropper. Stack can be lowered worst case by removing stem spacers and getting a no rise bar at the most extreme.
You can also look up what’s called the RAD number which is your height times some number should equal the hypotenuse between the stack and reach including stem length and bar rise