This is true. I have 3 AMEX cards enrolled in MR program. When I opened another (Business Platinum) they gave me another MR account number. However, I called and merged it to my existing account.

BTW, I got SPG bonus second time for SPG Amex Personal card. However, my last application for SPG card was two years after I got the first one. This probably is the reason.

I have another datapoint for Delta Amex Gold card. My wife canceled her card in January 2012 and got another one in September 2012 (when 70K offer came in). Even AMEX says that that would not give bonus points for those who had this card in the last 12 months, she got the full bonus. Probably, YMMV.