You've answered why the government guaranteed purchases of the vaccine & why production was parallelized. That doesn't answer the question why this changes the FDA authorization flow because in the flow chart EUA & full approval happen at roughly the same time & the claim is they have the same safety requirements profile (i.e. I am free to burn money & produce something someone doesn't buy - why would the FDA be involved).
Said another way, what was the extra steps that needed to happen between EUA & full approval?