These dudes are right.
Take an A power chord of A and E. That can be A major or A minor. it can also imply an inverted chord, like E sus 4. It could also be part of an Fmaj7 (f-a-c-e), F#m7 (F#-A-C#-E), C6 inverted (C-E-G-A), etc etc etc..many other chords. Without that 3rd you can't say, but you can usually guess based on the surrounding chords, bass line, or vocal melody. Usually one of those will play a 3rd. A and E power "chord" is a dyad not a chord, really. I guess debatable, but I don't have the energy.
All that said, if someone is playing A (I) - D (IV) - E (V) there's a good chance they're in A major, but technically you can't be sure based on just that.