Hopkins has a lot of real estate in Baltimore beyond what has already been discussed. The main hospital complex (which is huge), the medical school, the nursing school, and school of Public Health get the lion’s share of the security presence. Hopkins also operates another hospital on the city, Bayview. Then add in the undergrad campus and Peabody conservatory.
I am also not sure if the 1000 person number is limited to Baltimore. Hopkins owns a hospital in Howard County, and two more in the DC area. It also has the School for Advanced International Studies in DC.