>> The new veriScan system developed by the Metropolitan Washington Airports Authority—with guidance from U.S. Customs and Border Protection—scans the faces of travelers approaching the gate. The system then compares the photo to a gallery that includes images of that person—either their passport photo for U.S. citizens or the photo taken of foreign nationals when they entered the country.
So that's definitely not just the passports that have been checked into the flight- foreign nationals in particular may have entered the country at any time and place other than before boarding a flight at Dulles.
Which is why there are privacy concerns in the first place- because it can be used to track the movment of people across multiple trips.
you don't need to check someone's face against the entire
passport database, only against the passports which have
been checked into the flight
Sadly the gates are still slow and unreliable, even at that seemingly simple task.
* Admittedly for immigration rather than boarding [1] https://en.wikipedia.org/wiki/EPassport_gates
Doing so would be wildly inefficient. You need your passport to check in for your flight. When boarding the plane, you don't need to check someone's face against the entire passport database, only against the passports which have been checked into the flight, which is particularly important in terminals with shared domestic/international departures where entry is possible without a passport and the passport can be transferred to somebody after check-in.
But the main reason why it's anyway infeasible is because flights are rarely full of people of a single nationality; scanning peoples' faces against a wide-ranging passport photo database effectively means scanning their faces against some kind of global database of the faces of everybody from around the world. Assembling a database like that, apart from the ethical issues, is politically infeasible.
Remember - this system doesn't advocate for people to not need their passports at all when traveling. They still need them - just not after check-in. The question is whether the system is more or less accurate than a human being at matching passport photos to faces, and based on what we know about racial bias, the answer to that is probably that facial recognition is more accurate by now.