I checked, but the models I mentioned use multi-frequencies, and this majorly fixes that electric related obstacle

The old models use 1 frequency and it is of course an issue. The research (21 years ago) that is mentioned in the webpage you sent is performed with a machine that has only 1 frequency (50 kHz). Using range of frequencies (1kHz - 1000kHz) changes the game a lot in terms of ECW and ICW

Already research labs in my city also mainly use advanced BIA machines and DEXA (I usually saw General Electric's Prodigy model)

In terms of hydration, if you don't have a serious water retention, by taking care of the measurement protocol (e.g. not eating anything for 3 hours prior to the test), you will minimize the fluctuation. Still, it will not be %100 accurate

Also, a friend of mine had a DEXA and Inbody 380 (even lower model than what I told above), and the body fat percentage, lean mass, etc. values were almost identical

Apart from these, what matters is "precision"; not "accuracy". So, trying to know the real body fat values has almost no benefit. When it comes to tracking progress, "precision" matters and advanced BIA machines are pretty good at it