Doesn't dangerously low potassium present itself as flattened t-waves on ecg?

Yes: T-wave abnormalities are common with potassium disorders. A U-wave is also commonly seen with low potassium. This is a small hump right after the T-wave.

Yes: But flat T waves are very non-specific. Flat T waves may have many causes. "U" waves are another clue but could be seen with other conditions such as bradycardia. A T wave change (and U wave) is an ekg clue which must be verified by serum potassium level.