Without seeing the situation myself maybe what appeared to you to be cold wasn't at all cold to them and they were just fine. You said they still went under it later, so they know where its warmer and just didn't need heat at that time huddling together was enough at that time.
Not familiar with that brand you just bought but most brooder plate heights warm by the chicks touching it. Best way to use a brooding plate is to have it on an angel higher in front and lower in back so they can go where it is comfortable for them. Like I posted in your other thread most...