Technically, Biden is right. His family is not on trial in the #shampeachment. Trump is. I just hope somebody has the nerve to prosecute him in his own trial.
Your technical view is incomplete, it omits that Biden is central to the Democrat’s Trump abuse of power narrative, that the President somehow affirmed to President Zelensky to continue inquiries into corruption including Biden’s actions.
Hence, Biden’s corrupt behavior is central to the President’s defense.
Personally I would prefer to see criminal indictments of the whole gang, Biden included.