This is not generally true; it only applies to a narrow, specific class of programs. The "Early Release Observation" (observations chosen by the Space Telescope Science Institute) and "Early Release Science" observations (director's discretionary time awarded via a competitive proposal process a few years ago) do not have a proprietary period. However, the majority of the "General Observer" programs (these make up the majority of the time allocated on the telescope) have proprietary periods of up to 1 year. The proposers can request from a range of proprietary periods when they apply[0]. "Guaranteed Time Observations" (time awarded to the instrument teams) also have a proprietary period.
[0] https://jwst-docs.stsci.edu/jppom/jwst-proposal-level-inform...