Absolute tripe. How could an insurer ever know when an employee was on site on-shift or off-shift? In my (long) career employees have come into work on odd occasions - pick things up, speak to colleagues, bring the new-born baby in to show (maternity leave ladies). Not one of them has ever been turned away on grounds of 'no insurance'.
Here's a test for you. By law, your employer has to have Employer's Liability Insurance and they have (by law) to display the certificate somewhere prominant. Ask where it is and go and read the small print. I'll wager it won't say anything about this.