As far as I understand, the act prevents the prime minister from dissolving the parliament unilaterally, but in practice the parliament still only need a simple majority, by just passing a law stating 'notwithstanding the Fixed-term Parliament Act, the parliament is dissolved'. This happened in 2019 ([1], [2]).
Or it could have just repelled the act, which is what happened this year.
This can happen when there is no entrenched constitution and the parliament has complete freedom to legislate.