I think the instructional intent may be to prevent folks from putting an animal, especially a cold one, right under a heat lamp, or over compensating for the cold and delay by making the whole cage "hot". I know that this happens, the overheating of a new arrival, and can be just as problematic as the shipping delay itself.

We once shipped a baby monitor to a customer that was delayed a day and he thought it looked very dehydrated upon arrival. It is common for a reptile to dehydrate and stress during shipping, yes.

So he put it in a deep container of water to swim and rehydrate (not in a cage, just a tub of water). 2 hours later he found that the monitor exhausted itself from swimming (of course) and had drowned. He wanted a replacement. No.

The advice given above is sensible. Place your new arrival into a properly set up cage, but not directly under a heat lamp. Let them regulate their own temperature, as well as have access to clean water, and a hide spot.