The "east holdroom" is used for shorter haul DH1/DH3/BEH trans-border Jazz/Air Alliance flights. Flights have gates 200-299. Last I checked, this area is still being used (unfortunately). You take a bus to the satellite terminal once you clear US customs. This satellite terminal was also used back in the T2 days, but still exists.

I'm not sure why they don't use the in-field terminal (gates 500+). Maybe the gates don't work with smaller aircraft...