Can the USD in my SCB US Security Settlement Account be transferred into IBKR directly?

No, not possible.
I checked with the helpline before.

Only for online trading purposes.

I'm not 100% sure, but I believe that the US Security Settlement Account is only authorised for internal use, i.e., for transferring from USD to SGD (and vice versa) within your own SCB accounts. This means that I don't think it's possible to transfer money out of this account to another bank. I would think you might have better luck if it was say, USD$aver or FCY$aver account.