I'd say absolutely definitely written on the guitar.
The fact that the main piano part translates (assuming for a second that it was written on piano) so perfectly on to the guitar is just too much of a coincidence. The opening riff and chord sequence used are absolutely natural on the guitar.
It's in E, the mother of all guitar keys and the most guitar-friendly, and the F#m11/E and Emaj7 chords played are standard chords/voicings on the guitar which any guitarist easily slips into when fiddling around.
I think Bri's first piano-based song was Dear Friends.
Gullibility and credulity are considered undesirable qualities in every department of human life -- except religion.