After reading the answers above, my thought is the Webb is an observatory because it hosts multiple instruments, and a quick trip to the NASA web site confirms.
https://jwst.nasa.gov/content/observatory/instruments/index.html
“Webb’s instruments are contained within the Integrated Science Instrument Module (ISIM) which is one of three major elements that comprise the James Webb Space Telescope Observatory flight system. The others are the Optical Telescope Element (OTE) and the Spacecraft Element (Spacecraft Bus and Sunshield).”
https://www.nasa.gov/mission_pages/webb/instruments/index.html
The ISIM includes the following instruments:
—Near-Infrared Camera, or NIRCam – provided by the University of Arizona
—Near-Infrared Spectrograph, or NIRSpec – provided by ESA, with components provided by NASA/GSFC.
—Mid-Infrared Instrument, or MIRI – provided by the European Consortium with the European Space Agency (ESA), and by the NASA Jet Propulsion Laboratory (JPL)
—Fine Guidance Sensor/Near InfraRed Imager and Slitless Spectrograph , or FGS/NIRISS- provided by the Canadian Space Agency