This is a 'visible signs' condition. For theft to covered there must be signs of forcible violent entry to or exit from the premises.
Scaling a fence does not leave any forcible signs of entry. Using the door in the roller shutter to exit the premises does not leave any forcible signs of exit.
The insurance company are, I'm afraid, correct.
This isn't 'small print' - a visible signs condition in a commercial insurance policy is the rule not the exception, and has been for many many decades.
The purpose of the condition is so that walk-in thefts are excluded. If walk-in thefts were automatically insured, it would be open to abuse.