"as it repeatedly says mileage"
https://www.united.com/web/en-US/content/mileageplus/awards/default.aspx
Yes it says Mileage because
that's the parallel.
As for the evidence, since GTP is growingly dim-witted.
1) It's not attached to a specific car.
2) There's no measure unit.
Porsche equipped.
M: 258
Changes to Mazda. M: 258 remains.
3) Mileage EARNED
You have a
reward waiting to be claimed : Mileage icon.
3) It's a currency.
USE Mileage (notice the capital M), lack of unit, and symbol.
4) It's tracked separately from Distance Driven and Daily Distance Driven
And of course, it's placed next to Credits.
There doesn't appear to exist a 1:1 correlation between Distance Driven and Mileage either (spent Mileage not known).
And t's bounding box
on the main screen tells us it shouldn't surpass 9999 total.
Explained above and how do you earn it other than a reward of some sort as indicated, and if their uses only entail car "upgrades" I do not know.
Since there are other types of in-game achievements (saw a pump station one, will try to find it), it just may be that there are other mechanisms other than direct race prizes that may reward you with them.
Before this "upgrade/tuning" system using them, I thought they could be a specific currency for an users' maketplace.