That's true if the guitar is tuned to standard tuning. For alternate tuning, especially for open-string tuning for slide guitar, since it's tuned to fifths, the Root would be on the "E" string and the Fifth would be on the same fret on the "A" string.
For example, a Power Chord with standard Tuning (E-A-D-G-B-E) at the fifth fret would be:
A D G C E A
A# D# G# C# F A#
B E A D G B
...but with Open or Slide tuning such as Open D (D-A-D-F#-A-D) the same A power chord would be played on the 7th fret of both the "E" and the "A" strings.
G D G B D G
G# D# G# C D# G#
A E A C# E A
Keith Richards in particular like to drop the E string to a D and his power chords were on the same fret across adjacent strings.
True you can do one finger power chords if you use non-standard tuning but that’s a special case. I don’t think that’s what the writer is talking about.
Keith Richards is known for using open G tuning on certain songs and removing the low E string altogether. That is, he plays with only 5 strings.
Drop D tuning where you lower the E string a step lets you do one finger power chords, but I don’t think Keith Richards is known for doing that. Do you know a song where he uses drop D?