The most convincing part is the other places in the constitution which set up the presidency in opposition to Officers of the United States. However, it's far from clear cut, as people definitely did think of and refer to the president as being an Officer of the United States.
You can read her judgement here: https://www.courts.state.co.us/userfiles/file/Court_Probation/02nd_Judicial_District/Denver_District_Court/11_17_2023%20Final%20Order.pdf
The most convincing part is the other places in the constitution which set up the presidency in opposition to Officers of the United States. However, it's far from clear cut, as people definitely did think of and refer to the president as being an Officer of the United States.