Coverage numbers are not a perfect indicator.

No they are not, and it seems from looking closer at some of the source that a good amount of Class::DBI seems to be auto-generated (all that __PACKAGE__->mk_something stuff). Which makes it very difficult to check the test's code coverage. But that code is not even included by Devel::Cover in the coverage report anyway (see here). However, a good portion of the code that is checked by Devel::Cover is not tested.

I will agree that Class::DBI is surely better tested than the Devel::Cover numbers give it credit for, but that is not to say that it couldn't use a little more.