My account has not been charged for pre-order

Yes, when you ordered the iMac online, your card received temporary hold for that amount.

When the temporary hold expires, Apple will re-authorize again.

After your item ships, Apple will then charge your card.

The time it takes for the charge to show up on your card will be after it ships and after the product receives a tracking number.

Your bank may not show the item has charged, but don't worry, Apple already got their money...

My M1 iMac was pre-ordered on 4/30 and just yesterday it showed shipping from China with a UPS Tracking number and my card now shows the charge in full (today).

It really doesn't matter if something has happened to someone else since circumstances are unlikely to be exactly the same and there are over 3 million users here worldwide. So, if you have any concerns, the best way is to address it with Apple since we have no access to any billing or accounts anyway.

Well, it still does not matter what happens to someone else as it is strictly a matter of how Apple processes charges and how your bank does. Apple does a pre-authorization to make sure your card is good. That will show immediately. Apple then processes the charge when the item ships. Your bank will show it when they process it - these are all questions no one here can answer unless we worked for Apple and/or your bank.