Not too surprising, considering brainwave frequencies aren't so far from the tempo bpm or the speed of vibrato and etc commonly used in music.  When you think about the degree of focus it takes to play, and that the focus when playing a duet will have the beat as a commonality, one would pretty much expect the brainwaves to be affected similarly.

Now what would have been interesting would have been if they had also been monitoring some listeners, to see what % of listeners ended up In sync with the performer's brainwaves. 

But music does that.  "Sonic driving" of brainwaves via sound is a pretty well known effect.  It is part of what makes music sometimes actually kind of mind-altering or mood altering in a very real nuts and bolts sort of way.  Slower beats/tempos in music can correspond to brain frequencies that are active when people are asleep or relaxed.  Faster beats can relate to brainwave frequencies dealing with more alert frames of mind.  

A couple quick examples..

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Bach's Jesu would be in the brain's delta wave (deep sleep) or, if one considered the pacing of the notes with their durations, more like theta (dreaming) range.

This piece, on the other hand is largely in the beta range, which is what is what the brain frequencies are doing when you are alert, and doing analytical or logical thinking..

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For some people the effect will be more noticeable than others folks.  But the rather direct effect that is possible on the brain's electrical activity, especially when the sound is at concert volumes and the attentions focussed on it is part of what is cool about music and some of the mechanics of how it can change a person's mood or frame of mind while listening.