I’m with
Insurance4carhire to cover my ONTO evezy Zoe car hire excess and they cover courtesy cars as well. That’s why I wasn’t worried when I had a Model S loaner when my M3P hire car was in for repairs for three weeks.
Unfortunately they didn’t forewarn you, and it may be too late now, unless you could return the loaner, and borrow it again straight away, As it’s a Model X, it may be more than the maximum vehicle value that they will cover. I was ok with the S, but an X is worth a lot more.
Edit: Just checked and appears that they cover vehicles up to a value of £120k. If you’re really concerned, may be worth exploring the possibility of taking out the insurance one day, then returning the X officially, and signing the documents again to get it back. Check the small print to see any time gap needed between taking out the insurance and getting the vehicle that you want to cover.