Unfortunately this was on the spot upgrade as our luggage did not fit in the other vehicle I rented so I didn't have the time to research the facts but thanks for the suggestion guys, good to know about the Amex insurance. Can one get that after they rent the car or does it have to be activated beforehand?

chase reserve policy states

During this transaction, review the Rental Car Agreement and
decline the rental company’s collision damage waiver (CDW/LDW)
option or a similar provision. Accepting this coverage will cancel out
Your benefit. If the rental company insists that You purchase their
insurance or collision damage waiver, call the Benefit Administrator
for assistance.

I guess they got me with the similar provision, but i'll have chatgpt whip me up an appeal letter nonetheless and hope for the best and write an angry email to Sixt about their lying front desk agents.