There are different, competing philosophies of judicial interpretation.

The current SCOTUS majority champions “originalism”, which means that judges must apply the meaning intended by whoever wrote a law. This often turns judges into part-time historians, trying to delve into the minds and attitudes of people who are long dead. But in this case, it’s pretty easy to determine that the Reconstructionists who wrote the 14th Amendment had no intention of letting an insurrectionist become president. So “officer” clearly includes the president.

Originalism is in contrast to “textualism”, which means interpreting the actual text(s) of the law without worrying about what the authors intended. So “officer” must be defined according to how it’s used elsewhere in the Constitution, which (surprisingly) may result in something that the authors of the 14th Amendment never intended.

There are also other competing philosophies, but right now these two are in conflict. The interesting thing is that the SCOTUS has to side against Trump if it actually believes in originalism (as opposed to using it as a pretext for their antiquated ideas).