[analog31]

It is and it isn't. All of the violin family instruments are tuned in fifths, but the strings have limited tuning range and would either break or produce no sound if you tried to force-fit a single tuning onto all of them, e.g., tuning a violin like a viola or vice versa. The double bass is its own beast, usually tuned in fourths, but in fifths is virtually unplayable without injury by mere mortals. And fifths require careful choice of strings.

You still have to read the clefs.

A remote possibility is that the person was a guitarist or bassist, and tuned the instruments in fourths. I have a friend who studied classical guitar as an undergrad, and music theory in grad school, while earning a living as an electric bassist in R&B bands, then became a theory professor, now retired.

Disclosure: Double bassist.