Is 11th Generation Intel® Core™ i7-11390H Processor (12MB Cache, up to 5.0GHz with ITBM3.0) compatible with LInux Ubuntu 20.04?

This will work absolutely fine, its just that there is no agreement between Dell and Canonical for testing for this specific chipset (and even then, I expect that if they are both Inspiron 5510's they are both covered with the Dell advisory. These chips are pretty much identical except for the cache and CPU speeds - almost certainly they come off the same line and are just binned differently, as is common practice with Intel CPU's.

Another take on this - the CPU is almost certainly supported by Linux and thus Ubuntu. The certification of a device typically looks at the "other" elements of it, like the touchpad, network card and other drivers. These will be the same on all Dell 5510's and thus will work fine with Ubuntu.