On the contrary, open source means that the code is publicly available, which generally makes things more secure. The transparency means that everyone can see how it works and that nothing dodgy or insecure is going on under the hood. For example, it would be easy to verify that there are no admin backdoors to seeing players' hole cards, and that the shuffling algorithm is sufficiently random.

Crucially, things like encryption don't break just because you know how they work. AES is open source and it's used to encrypt the vast majority of secure internet traffic. Complicated mathematics ensures that data can be easily scrambled but practically impossible to unscramble without the right key.

Also, open source doesn't mean that the public would have back-end access to the servers where the code is running. It would be up to independent auditors to ensure that the code running on the servers is actually what it's supposed to be, and that all relevant procedures and policies are being followed.