How should I judge this report about four alien species?

I’d start by separating the headline from the underlying claim. Who are the researchers, what exactly did they examine, and is there a report or other material that can be checked? A description of sources who fear speaking publicly is not, by itself, a way to verify what they say.

For a claim this extraordinary, look for evidence that independent specialists can inspect, along with clear details about where it came from and how it was analyzed. If those basics aren’t available, the careful conclusion is that the claim remains unverified, not that it has been proved or disproved.

I’d be cautious about treating a report of what unnamed sources allegedly said as confirmation. The useful next question is whether the story points to specific documents, identifiable researchers, or evidence that can be independently assessed.

If it doesn’t, there isn’t enough in the claim as presented to establish that any craft or species were recovered. That doesn’t settle what might exist; it just keeps the conclusion in line with the evidence available.