Is not confusing
Let's start with
Part 91 can take off in zero visibility (not the smartest move).
Now on part 135.
You have your ops-spec and should said the lowest you can go
Is 500 yo are allowed to go 500 (you have to meet the requirements)
And because your ops-spec are approved by the Faa you're good.
If you don't have the lower of standard (because the Faa didn't approved)
You follow the 10-9