Ok, here´s the promised answer. Amex wrote me that a one car class upgrade "based on availability" means that SUVs are excluded from this because Hertz excludes SUVs from upgrades. At least in german I would suggest Amex to copy the whole text from hertz on their site.
The answer from Hertz is not worth to talk about - they send me a link to the Unlimited Choice site which is blocked for german IPs.