From your link:
The allegations are not "enough to convict [the president] of bribery" in a court of law, Napolitano says, "but it's enough to allege it for the purpose of impeachment" since impeachment is "not legal [but] political." The judge adds that while he thinks impeachment is "absolutely constitutional," it is also "probably morally unjust." Besides bribery, he lays out four more likely articles that he thinks House Democrats will bring against Trump.
"The second charge will be high crimes and misdemeanors, election law violation," says Napolitano. "The third crime will be obstruction of justice. The fourth will be interference with a witness and the fifth may be lying under oath."
Napalitano is not saying that Trump did anything wrong, he's saying that what Trump did could be impeachable, since impeachment is political and not legal.