My experience indicates that they do not check. I saw this thread and applied for the Sixt Gold Card. However, I made a mistake in the application process -- I did key in my LH FTL number but somehow managed to change the program name (from the drop down box) into TAP Air Porgugal. I got approved a few days later, nevertheless. Had they done any checking, they would have realized that a) the number did not come from TAP and b) I am not a TAP FFP member.

But the card is in the mail...

Cheers,
T.