That's not what's at play, here. What's at play is setting a precedent that a President could be held criminally liable for actions committed while in office, after leaving office. No sitting President is ever going to willingly set that precedent, and in fact, they're going to actively resist it because the precedent alone means a president sits in office with Damocles' sword hanging over their head (as they damn well ought). If Trump's liable for shit he did in office, the rule of law dictates Biden and subsequent presidents become potentially liable four, eight, or more years down the road.
Case in point, as the article points out, when the Obama administration outright refused to investigate Bush administration officials and Bush for malfeasances committed whilst in office. And, by incredible coincidence, when the tough-talking Trump administration refused to do the same to the Obama administration, even if only as a partisan ploy for which the Trump administration later proved itself eminently capable and willing.
Nixon was a shot across the bow to future presidential administrations that the rule of law does indeed apply to chief executives and their administrations, and even then, Ford pardoned Nixon. Between the S&L crisis and Iran-Contra, great lengths were taken during the Reagan administration to set precedents that elected executives and appointed officials could and would not be subject to prosecution for acts committed in office, and the Bush II administration worked tirelessly to insulate itself from investigation through endless appeals to need for secrecy and national security. No sitting president or president-elect is going to commit to reversing the precedents set between those two administrations post-Nixon.
DoJ's aren't actually independent, you know that, right? Those are presidential appointees and inherently political, and presidential appointees are, unsurprisingly, going to act in accordance to their appointor's agenda. That's how and why they get the damned appointment in the first place. If Biden says he doesn't want Trump investigated, the "independent" DoJ isn't going to investigate Trump, because Biden's appointees are going to "independently" do what Biden wants and doesn't want because that's what they were appointed to do.
Biden's commentary is political doublespeak, to dodge accountability and enable defenders to rationalize and shift blame just as you're doing now. I strongly suspect you already know this.