This is not I'm saying. You are responsible to calculate G's.

What that warning says is that the building code may be not proper fitting with the vessel behavior.
However, you can evaluate the fundamental period of your vessel and apply a judgement as building code make sense for your case.

The main question would be- IMO- the R value. Adopting R=1 is over-conservative but a Client would ask for it. You can adopt other value, but here you have no support from the building code as it is not referring to your case. Your Client would ask you (or not) to document your choice.

R value is the ductility factor and a building code would associate this factor to the structure's capacity to develop some non-collapsing plastic hinges, which is not your case. In the same time, the foundation-soil interaction during seismic event is an argument for R>1.
I mean R value is an sensitive subject and depends on many factors, case by case. It may be a subject to deal with Client or you can take the responsibility to adopt a value.