From the image, the expanded Tekong obstructs approximately 2/3
the width of the river mouth.
During a heavy downpour, if the river swells, floods the bank and
spills inland, then the reclamation is the cause.
If the river swells but does not spill inland, and the water level
rises from within the city, then it's the drainage system that
cannot cope.
Of course the experts' presence will be required to study the
situation when the next heavy downpour occurs. Will require
helicopter and boats and money too.
Theoretically, the accusation by Malaysia is valid.
BTW, TEKONG means obstacle.