"During manoeuvring for an approach,when the situation dictates an earlier than normal speed reduction,the use of flaps 10 with gear up is acceptable."
Note that this statement says that
using the flap to achieve a lower speed while manoeuvring, while keeping the gear up, is OK.
That is a great deal different to "extending the flap at high speed, to increase the drag in order to slow down" - which is what you were attempting, and the Captain was warning against.
If you need drag, because you are fast - add drag.
If you need lift, because you are reducing speed - extend the flap.
Your Captain was correct.