Why is my iphone 8 plus broken after being submerged in water for less time and less depth that it says it is able to handle?

The phone is water resistant to a specific standard that is tested under laboratory conditions. It is not intended that it be submerged in everyday use, and water damage is not covered by the warranty.

Unfortunately, consumers don't seem to understand how water resistance ratings work.

That applies to both electronics and things like watches.

This graphic from Casio is a good illustration of what it means for a watch:

Now... consider this: The same type of testing is used for both watches and electronics like the iPhone.

The iPhone is tested to between 1 and 2 meters depending on the model.

Now look at the chart above.

You'll note that watches tested to a depth of at least 50 meters are considered appropriate for use in shallow water.

The same applies to electronics.

The iPhone is not waterproof.

Your device has been destroyed by the submersion and can't be repaired.