The issue that John Neilson is referring to in “how to accurately anchor the floor plate” is one that affects any FEA model. The model requires that the “floor plate” be restrained in some manner to keep it from flying off in space when a load is applied to it. Should he just extend the floor plate an inch or so beyond the cage baseplate, and fix the edge there, or should it be 3” or 10” or what? No matter what you do, you will get results for that particular model and those results would not necessarily be indicative of the result obtained for a different model. You eventually come around to the conclusion that to really know the answer one needs to model a substantial part of the belly pan, at least to the extent that it reaches some a more structural portion of the frame.
And when he talks about having a doubler “under it”, he clearly means under the floor pan, not the cage baseplate.