Found another article explaining Interbank Interest Rate Determination in Singapore. Published in 1999 by the Monetary Authority of Singapore (MAS), it stipulates that
Only changes in US interest rates or market expectations of future movements in the exchange rate have a significant impact on the domestic interbank rate.
That should be enough proof that SIBOR and SOR tracks interest rates in the US. This chart of the US 3 months interbank rate may be a useful resource.
