Found smoking gun here. I have pdf'd this page for future reference:
https://www.mbfs.com/mbfsr/en/misc/creditCard.do
Read note 8, specifically:
It will typically take from 6-8 weeks after you reach $5,000 or more in eligible purchases each year to receive your certificate from Mercedes-Benz Financial Services
Seems pretty clear to me and think they will have to honor that