Adding this now as an answer instead of a comment:
The article refers to Enrollment on Arrival as "walk-in". However, the enrollment on arrivals don't happen at the regular enrollment centers, but rather directly at the immigration booths which you only can reach by arriving on an international flight.
Example: In JFK the enrollment center where you can make appointments, is in terminal 4. However, enrollment on arrival is possible in terminals 1, 4, 5, 7 and 8.
Independent of EoA, you can of course just stop by the enrollment center to check if they have capacity for a walk-in (e.g. because of short notice cancellations), and nobody there asks if you had an international flight (source: own experience in February 2019).