Yes it is true!
I recall from the CPL and ATPL theory (and this is very theoretical!)that the interference indeed happens at multiples of 360.
A bearing from a VOR is calculated from the difference in phrase signals being received from from the VOR reference signal, and a bearing signal.
This difference, received in your VOR, is then translated into a Bearing. Obviously to a maximum of 360 degrees.
Anything more and you will have to look it up the theory books!
Hence you seeing this interference at 1800 rpm (360 times 5).