It is the duty of gambling platform itself to be robust and proactive towards preventing multiple accounting bonus abuse, if there is any. When you took his/her bet (does not matter if bonus or deposit) you must cash him out .You cannot simply hide behind the multiple account excuse (quite prevalent in crypto community though).
I am with OP on this matter. OP open a flag against them with scam accusation.

Note: Website looks like created in a hurry, you can guess the reason.

Do update us about the matter OP if resolved because people open scam accusation but do not care to update.