Have you found anything else about the scam activity? I am not familiar with the gambling site but I do not want to conclude the gambling site as a scam. The email might be sent for the account you have created or mistaken even many people who may create an account by taking the email of others to spam or waste the time of others.
Yes you could be right but from op as he may have explain that he never remembered making an account with that site, and of a truth there are some people are who are there collecting mails to sell to some site for person gain. Let say when an email is associated to a particular site or airdrops they automatically archived the mail to their system to sell out to some new site that wants to send random or marketing mails to people, maybe through they might get some people who may decides to follow the mail and make account with the site to start gambling with them. The thing is we should be extremely careful with the way we received uncalled mail from random sites.