I believe it would be up to the Code official to interpret this. That is, if you do not have the test results to show that the mud, silt or fill has a certain bearing capacity, you would have to demonstrate to the code official that the structure is lightweight, and that the mud, silt, or fill is adequate to support it.
In our case we have determined the fill has adequate capacity (1500 psf)but that seem to me to be independent of twhether the building is "lightweight". The actual pressures are in the range of 0.6 to 1.2 ksf.
1804.4 states that the presumptive values in Table 1804.3 can not be used for mud, organic silt, or unprepared fill. A loadbearing capacity can be "approved based on test results," and used as the presumptive loadbearing capcity for the project. If you have determined the fill has adequate capacity, then it is an independent issue-the lightweight structure provision does not apply. The bearing capcity can be approved based on your tests.
