According to the usage I see in the tests, this means not "does the DBMS support a Boolean type" but rather "does the Django backend define a unique field type for BooleanField". I think this is the wrong semantics, and the right semantics is needed by anyone who needs to address booleans in raw SQL.
AFAIK, both Oracle and SQL Server backends define "tiny int" columns for BooleanField, but neither of the DBMSs supports the keywords True, False, or Boolean.