That is the only way to test the oil level in that automatic, The extremely critical point to note is the oil temperature when the bolt is opened. Because when the oil gets hot it expands a lot.
This expansion is the same reason regular old automatic had a dipstick with cold and hot marks - the expanded oil sits as "higher level" in sump.
Now keeping above in mind, if you open the drain bolt on fully expanded ATF to check the oil level, you will drain close to a litre of it, this makes the gearbox oil level actually low because when it will be cool the oil level will be extremely low as the oil is now missing.
If you open the drain bolt at required temperature your level will be correct - Just follow what the manufacturer tells you - they built the gearbox,