It’s more looking at the other musician’s fingers in the case of a string quartet (or when I played in rock groups as a bassist, I would have my eyes on the drummers hand and foot on the kick drum pedal). Likewise, when I’ve done orchestral work, I generally had one eye on the music, one eye on the conductor and one eye on the principal's bowing.

Interviewing the Guarneri quartet is about the most extreme example they could've found in this regard. They are one of the most longstanding and in lockstep chamber ensembles on the planet. Most chamber groups engage in enough eye contact to get them through changing elements of a piece, like ritardandos, accelerandos, fermatas, etc.