I see this stated from time to time, for example in an answer to Can large airliners do aerobatics such as loops?
A barrel roll is one thing; they can be performed at constant load factor of 1 G (assuming sufficient control authority).
In a properly executed barrel roll, however, I do not think there is any point in the trajectory where the vertical velocity is constant. In particular, the entry and exit of the maneuver require transitions from level to ascending, and from descending to level trajectories, respectively. In these two cases, at least, I cannot see how a load factor greater than 1 can be avoided.
This IAC guide states a maximum in the range 2.5 - 3 (and a minimum of about 0.5), though it is probably not intended to be a minimum feasible value.