Deposit a lot of money to buy credits and I would think that would qualify you as a top customer no matter what your skill level is.
Not knowing the player or the history, did he/she show a heavy increase in game play around the time the invites were sent out? Maybe WGT was taking that into consideration.
I believe I got my invite because I've been a member for over 5 years. Maybe this account is someone else who has stuck it out for just as long, has played honorably and just needed a new account for a legitimate reason.
"Top customers" means just that. They're not all legends and shooting the lowest scores. Given the large number of things that can cause one to be considered a top customer, I don't see a problem here at all.