The 90 days fiancé visa rule applied for US. citizens. However, the fiancé visa UK works differently in the United Kingdom.

A fiancé visa is granted for 6 months to allow the applicant to get married or civil partner in the UK on a fiancé visa within a period of 6 months upon entering. This includes giving Notice to Get married at your local marriage registrar. The good news is that the non residing applicant is not required to leave the UK and can post marriage or entering into a civil partnership can make a new application to the Home Office to continue to remain on a Spouse or Civil Partnership visa.

Original Source: https://www.quora.com/How-does-90-day-fianc%C3%A9-visa-work/answer/Sukhdev-Singh-1777