My email also mentions the 500, but I do see the confusion now, not to mention that the entire offer actually makes no sense. Here's the verbiage:
Enjoy the comfort and superior service that Acela® travel has to offer and earn an additional 500 Amtrak Guest Rewards® bonus points for each trip. Plus, on all other Amtrak® travel, earn 100 bonus points for Coach class, 200 bonus points for Business class, and 400 bonus points for First class trips.
Note that the first sentence says 500 AGR points for each trip on Acela. No mention of class. So it would seem that a ride on Acela earns 500 points no matter what and without regard to class.
The problem with that is that, at least according to Amtrak's current definitions (AGR's may be different), the only place that one can ride in First Class is on Acela. Therefore if we assume that they're not going to pay for a 900 point bonus, then the entire bit about getting 400 bonus points for First Class trips is useless.
Amtrak used to consider sleepers as First Class, but now it's called Sleeper Class. But of course it is possible that AGR still considers sleepers to be First class. So who knows?