I would address the letter to the "they" mentioned in the quote above, which I think you are referring to Scoot. The instruction must have been missed out somewhere between Scoot and their ground handling agent.

I have always seen ground handling agents waiting for the passenger with the wheelchairs ready when I disembark in Changi, never the other way round. In fact, the agents would already be waiting outside, even before the plane pulls into the gate.