'Then ground down' is a much bigger statement then you may think. If we look at the particle size of flour via historic grinding processes what did it look like? I'm sure they were far less uniform and in general larger than what we have now, which should lead to differences in the glycemic index.

CICO would have you believe there is no difference in 100 calories of coke and 100 calories of raw spinach, but the coke is going to have nearly instant uptake and the spinach will take hours to digest.

In the US it's allowed [1] to add bromate to the flour, which helps with the texture apparently. Brominated flour probably counts as highly processed.

1. https://www.kingarthurbaking.com/pro/reference/bromate

Flour in a supermarket isn't just milled, it's milled finely and bleached and so on. I wouldn't know where to get 'just flour'.