He DIDN'T interfere with the work of the DOJ. If anything, he gave them permission to go ahead and investigate.

That's not the point.

In his pardon statement, and the subsequent explanations from the press secretary, Biden claimed that the prosecution of Hunter was infected by raw politics. He's claiming that the DoJ was affected by politics. That Hunter was prosecuted because of (presumably) Republican political pressure on the DoJ demanding that outcome.

But that's....a really bad thing to say about the DoJ. He's claiming that DoJ is not independent. That it responds to political pressures. That it is sufficiently pliable to political pressures that even when they come from the opposition party to the President, it is vulnerable. If the DoJ can't resist political pressures coming from the minority party, even as against one of the most politically connected people on the planet (the President's own son), how could it possibly be independent of the political pressures brought by the President's own party against the President's opponent?

IOW, if the DoJ could be steamrolled by Republican Congresscritters into wrongfully prosecuting Hunter, how could it possibly avoid being steamrolled by Democratic Congresscritters into prosecuting Trump? If the prosecutorial decision for Hunter couldn't be protected from Republican influence, how could anyone possibly believe that the prosecutorial decision for Trump was protected from Democratic influence?

That's why Biden should never, never have tried to pin the reason for the pardon on something wrong with DoJ. But he did. Because he didn't want his legacy to include using the power of the office for personal reasons, so he threw DoJ under the bus.