I'm wondering if the Amex site is looking at location or something to customize their incognito offers. So far the link you posted is the only way I've been able to see that offer.
Doubtful it's location based. In the incognito browser, did you Google 'American Express Hilton 75'? I used Chrome incognito and I found the link on uscreditcardguide.com
Is it possible to check the signup bonus amount of the card after receiving it?