When he first demonstrates the little chords he strums across all 4 strings.
Then he demonstrates just playing the 2 notes on the 4th & 3rd strings with a pick and then by plucking them with a pick and middle finger.
When playing the 2 note "chords", he's not pressing down on the first 2 strings, but
his finger may be still lightly touching them to mute them out (at least for the 2nd string).
(When he first demonstrates them in A he puts more "arch" in his fingers to emphasize he's only playing those 2 strings.)
For the slow and fast play-alongs he is strumming the all 4 string version, so he is pressing down the barre on the top 2 strings.
At no time are the top 2 strings played open.
They are either played using a barre in the 4 string version or muted by releasing pressure in the 2 string "plucked" version.