I thought that you had to pay for something at a resort. How did you get a resort credit for a club fee? What Resort?
With Amex (and some other cards, like the Chase Sapphire Reserve), the category credit can count for things that one wouldn't think it should, LOL.
For example, the Amex and CSR state you get $XXX for "airline" credit, and usually specifically exclude Airline gift cards. BUT, there's all sorts of data points on Flyertalk that folks can buy $50 airline gift cards (depends on the airline, what denomination works) and it gets coded as an "airline" credit, so they get reimbursed for that.
I'm guessing it's the same thing here. Quite possibly when HGVC processes the club due, they're coding it in a way (maybe it shows up as like: Hilton xxxx) and that triggers the Amex credit. The process is automated.
I would think that (unless HGVC codes only the club dues this way, vs. the MF), that your MF would be elilgible as well (up to the dollar amount) for the credit.
But I don't have the Amex Hilton cards anymore after the big devaluation last year. I switched to Starwood/Marriott.