Good point and really it is a matter of long-term vs. short term. I thought along the lines of long term in which the promotion is repeated. This is not neccesarily the case and the worst case detailed below still nets to 15,000 -16,000 miles all for $80.

Diners and BA have run this promotion every summer since 1997 and Diners is very efficient at processing applications, so everyone should be able to get 3 to 4 months worth of the promo this year and the rest the following year. At worst you will get 6,000 to 8,000 BA miles plus another 9,000 to 8,000 on ANY airline including AA. Recall the Diners Club offers the transfer of miles with virtually any airline.

That is the WORST CASE SCENARIO 15,000 -16,000 miles for $80.

Given my experience, that Diners will do the BA promo again it is indeed the 24,000 miles posted.
Hope that this helps, but let me know.