BaseEstimator provides the estimator tags machinery. However, if an estimator does not inherit from this base class, we should fall-back to the default tags.
For scikit-learn built-in estimators, we should still rely on self.__sklearn_tags__(). get_tags(est) should be used when we are not sure where est comes from: typically get_tags(self.estimator) where self is a meta-estimator, or in the common checks.