If my salary is credited into a POSBank savings account, and I manually transfer my salary into my DBS multiplier account, does that count as salary crediting?

No. You can actually see that there is a difference. If you look at your POSB account, the day that salary comes in, beside the amount you will see SAL. That is salary crediting. But when you transfer money, you can see that the abbreviation is different so it is not. That is how the bank assigns.