These videos are designed for guitar beginners or those with only limited guitar knowledge. They use simple chord shapes as much as possible and do not necessarily represent the way in which the song was originally played. They are designed as practice tools to supplement your other learning methods. Information is provided on screen for both right and left-handed players!
Marry You only uses three chords, but it's a fast song. If you're not yet comfortable with strumming then don't worry about doing that for now, and just work on getting the chord changes happening at the correct times. I've simplified things by putting a capo on fret 3, whereas Bruno himself plays the song live using barre chords.