It wasn't anonymous, it was confirmed by committee members that deal with this topic. The White House/DoD has also offered to brief the previous administration

It wasn't anonymous, it was confirmed by committee members that deal with this topic. The White House/DoD has also offered to brief the previous administration members on the topic. The entire premise is that the previous administration just didn't know about it because they only just discovered this occurred (I assume by putting together multiple pieces of evidence from the various intelligence apparatus.) Keep in mind there are various new programs dealing with this exact topic to monitor unknown arial crafts over US assets. So of course, the previous administration could confidently deny they were briefed on it, because no one has even claimed the previous administration was ever briefed on it.