I'm not sure why they would think that.
In the 5x 300mm configuration, there are four rockets arranged in a 2x2 square pattern with the fifth rocket in the middle of the square.
In the 4x 370mm configuration, there are only four rockets arranged in a 2x2 square pattern.
That image is clearly taken from a downwards facing angle looking up -- if it was the 5x 300mm configuration, then you would expect to see the fifth rocket between the "two" rockets of the "square" without the ability to see through a space between them through the center.
Whereas in a 4x 370mm configuration, as we've all seen from the PHL-191 on parade, it has a clear space between them through the center.
Frontal view of AR3 export, showing the five tube 300mm configuration and the four tube 370mm configuration, and how there's no center space on the five tube 300mm configuration as it's occupied by the fifth rocket tube.
The PHL191/16/whatever it is, in service with a dual 5x 300mm (right side of vehicle) and 4x 370mm (left side of vehicle) configuration