Yes, but the 30 days is in a situation where your number was not on the ticket. If your guest rewards number is on the ticket I believe you have 90 days to request missing points to be added to your account.

Here are the rules:

How do I receive credit for tickets purchased without using my membership number?

To request credit on a previously purchased ticket, send the ticket stub with your name and membership number to: Amtrak Guest Rewards, P.O. Box 1762, Minneapolis, MN 55440. Requests must be received within 30 days of travel. Retroactive points will not be granted for travel occurring more than 21 days prior to enrollment.

If points are missing where your ticket was purchased with a rewards number, you have 90 days to request credit. In your case, you know the points won't show up since you didn't give them your number when you bought the ticket, so you don't really need to wait 3 weeks.