yep, it could. If the rock wasnt out of water too long you probably wouldnt have had any die off. For example, I just started a 90g tank and half the rock came from my 125g. I had no spikes or readings on ammonia or nitrites but the live rock had enough beneficial bacteria to support some fish being added. I've been testing every day and still no spikes even with the fish. So it is possible to insta cycle.
I should have added ammonia to be be sure but I've done this a few times so I was pretty confident.