I think the 'second half of May' may have been simply because they did not know with absolutely certainty when their supply chain would be totally ready. Given chip shortages and the impact of the pandemic in their suppliers, this probably wouldn't be surprising when they announced the new iMacs.
By the time preorders opened, it had become a little more clear, but even then, the fact that customers were (and are) being given a delivery target several days wide, suggests that it has been logistically problematic this time around.
But I would expect those being notified of delivery prior to 21st May finding their shipments held. It's entirely possible I will be proved wrong, but that's been Apple's practice in the past, and UPS and FedEx have seemed happy to comply.