I think that what you're saying implies that FLOAT would be the correct type for BLENDINDICES.
I think so, but that does then force us to add that extra interface, whereas we could otherwise (always? usually?) get away with guessing UINT for blend indices and FLOAT otherwise. Which is annoying :-/
(I did say that I ran into Vulkan validation errors, but on second reflection I'm not sure if I have, *except* for BLENDINDICES.)