Nope, he is right. In that it is a problem of backpressure in the exhaust.
An open exhaust pulls back fresh air into the exhaust, where there is leftover fuel, which ignites and bangs. Not good. Also because the lack of vacuum you get less torque in lower rpm. Also the lambda sensor gets false readings with the fresh air from the outside. It switches the EFI to lean the mixture.
The Cat is quite ideal for this purpose, it has realtively low resistance for the hot gas leaving the cylinder, but creates a lot of turbulence for the air that is sucked back into the exhaust. So if it was not already invented, it would be a fine (and expensive) device for tuning open exhausts. A ceramic insert for open exhausts with optimized floating design and platinum coated.
To get rid of the bang in the exhaust you should either lower or enhance the AFR in low rpm range, where the machine pushes. Enhancing AFR works much better.