I think it's a far better metric for the popularity / activity on the site than overall user registrations. You can imagine a regular player that registers a new account for every gambling session to hide his identity or because he doesn't understand what provably fair is and succumbs to fallacies, e.g. that the casino makes him lose after luring him in. This can be seen on other sites. So he constantly increases the registered users but it's just 1 daily active player (or 2 or 3 depending on how many accounts he registers).

The bias in the other direction is also possible: A regular player that only registered one account. So even if he plays regularly, the overall registered user statistics doesn't treat him differently than a player that only played once and never came back again.