This tune is 100% modal, and it uses a single scale from beginning to end.
Now, please note that "modal" doesn't mean "using one of the seven major scale modes". "Modal" describes any music which is built around a scale -- any scale -- rather than on the combination of chords and melody.
For example, Indian classical music is modal, while Western pop, rock, and classical music are not, being based on harmonic progressions. (Exceptions exists but are vanishingly rare, e.g. Pink's song "Get this party started", is a rare example of a modal pop song. You'll notice that there are basically no chords, it's just a B minor scale)
Blues and bluegrass may fall in either camp -- if you play a blues in which the chord progression is in evidence, then the harmony dominates, and the melody and improvisation is forced to follow the chords. But if the chords are not used, or limited to a single chord, then the improvisation becomes an exploration of the scale, just like in the video above. And then what you have is essentially a modal tune.
Similarly, most Jazz is not modal, as chords are central to most Jazz tunes, but if you leave chords aside, or use just a single chord for an extended period, the music takes a modal flavor immediately.
Bottom line, whether the scale has minor thirds, or major thirds, or both, is not the point -- if there is no chord progression, and the star of the piece is the scale itself, that's what modal music means in general.