WWP and reserves

I played a game today and had my Archon with a WWP jump in a venom with some incubi, I was going to bring my wyches and cronos though it. once opened. My opponent said i had to roll for reserves and if the roll was successful they had to come on regardless if the WWP had being dropped/opened or not. In this case it hadn't so my cronos had to come on on my table edge so far away from the fighting he was useless, totally defeating the purpose of having the WWP my wyches came through it however. Is that correct about reserves and WWPs?

Yes it is correct. That's why you need to drop it on you first turn (rather annoying). In the old codex you could keep things in reserve until you dropped it.