But only until a President shall have qualified. Not permanently as is the case when a legitimate President is removed or dies.
Since the present Congress is dominated by Democrats, Biden would be President for all intents and purposes until such time as Congress acts.
That would mean that Biden's signature would be adequate to make legislation into law. Though there might be an asterisk by his name, Biden would be the next President in the history books. An "acting President" is a President. He is one who is authorized to preside and act.