Presidential immunity, US Supreme Court

The Supreme Court ruled that the president has absolute immunity from criminal prosecution over official acts. This covers some of the things he is accused of doing to steal the 2020 election.

I am puzzled by this statement:

his prosecutors will not be able to introduce as evidence any acts deemed to be official and struck from the case, even as contextual information for jurors to show Trump’s intent.

Why should facts be prohibited from use as evidence to prove a crime not covered by immunity, merely because they are not in themselves criminal?

The attempted explanation at the end of the article did not make sense to me.