I don't think I would have had time to carry out the necessary performance iterations from 600', so if I were to reduce flap in that situation it would be a 'gut feeling' move only. Maybe they did? Certainly despite what you may read from 'the experts' on the other (amazing!) forum, reducing a landing flap setting will not reduce the lift much but
will reduce the drag, therefore it will possibly benefit. However, see Rule 1
Gear MUST be left down to absorb the energy of impact in that situation. Raising it MIGHT allow a stretch of the glide, but airliners' drag normally increases as the gear goes up and down due to various doors opening and shutting, so it would need to be done at sufficient height to make a gain, and 600 is not that!
Having said all that, the advice you have been given is spot on
