Graber bike rack installation on Honda Accord
When properly installed these type of trunk mounted racks do not actually rest on the bumper. 

The lower arm should be positioned slightly above the bumper and bottom edge of the vertical part of the trunk lid. The weight of the rack and bikes is actually supported by the top straps. 

See the pictures at the Graber web site (not sure if it's exactly the same product, but it shows proper installation.)

https://www.graberproducts.com/product/guardian-elite-2