Bottoming the boat was one way Kaleun's used to evade the enemy. If they could effectively get the keel on the bottom it made it indistinguishable from the surrounding terrain (excluding shallow water). The only clue the DD above would have is if their hydrophone operator picked up the sounds of the keel touching the bottom.
Imagine filling an empty jacuzzi with ice (enough at least to cover the seats and sides). Obviously any boat moving around inside the area is gonna stick out; now imagine if that same boat plants itself of the side where the surface is irregular shaped. Sonar of the day would have been unable to distinguish between the ordinary ice cube or a 30" cylindrical one sitting on the ground. Even in Das Boot when the ship bottoms out the DD make no attempt to further depth charge it. Sonar couldn't distinguish between whether an object on the bottom was a rock or ship unless it moved.
What had initially infuriated me was that I managed to bottom my VIIC in Gibraltor (southern side) at 250m+ (guestimatted at around 300m). Using the external cam I watched a pair of DD's depth charge me with pinpoint precision at 1/4 of a mile depth, simply ridiculous.
For that matter, weaponry that precise didn't come into service till decades after WWII had ended.