Regarding the SSO option, it’s very easy to accidentally click on “Sign in with Google.” It has happened to me several times on other websites too.
As you mentioned, you never played or used any bonuses on that account, so this really feels like an excuse to deny your withdrawal.
I'm not really that heavy into gambling, but I can say this is also the reason why I stopped playing on BC.Game. Since I have multiple emails logged in on my Google browser, it really happens sometimes that you accidentally log in using a different Gmail account. So the moment I mistakenly logged in using another Gmail account, I stopped playing on BC.Game because I was worried something like this might happen even if you're not trying to cheat their bonuses.
This has also happened to me in another casino where I was logged in using a different Gmail account instead of my main one. But after it happened, I informed the casino that it was an accident that I unintentionally created or logged into a second accountand I requested that they delete the second account.
Maybe if the OP had done something similar and explained the situation in advance, especially before winning such a large amount, this might not have become an issue.
I never did anything remotely, as stated few times.
When I registered on BC.game initially (my active account back then, advanced KYC verified), I've signed up by typing my details (email + password) and not Signing up using Google SSO. At one point after using that account, I tried signing back in (not signing up) to it using Google SSO button for it. It created whole other account. Same e-mail address as my original one.