So can you fly 300kts while on the RNAV STAR within 40DME prior to POSUB?
Yes, if you are landing at WSSS. Re-read the paragraph from the AIP.
All arriving turbo-propeller and turbo-jet aircraft are to fly at not faster than indicated air speed 250knots when within 40NM from Singapore Changi Airport or when at or below 10,000ft,
except all arriving aircraft into Singapore Changi Airport shall comply with the speed restrictions depicted on the transitions and RNAV STARs. Further speed reductions will be regulated by ATC as necessary.
If you are arriving to WSSS on the arrival, you slow at POSUB not at 40NM.