Honestly...I'm going to give it at least a year before I try and read too much into Biden's intentions from these kinds of comments.
I don't think it can be so easily overlooked that part of Biden's entire presidential platform--which just won--was focusing on unity and healing and trying to win support by convincing people that there's still room for those things in modern politics. I have my own disagreements on the practicality of that, but either way, it was the platform he ran on, so to publicly target Trump right now would be an awfully quick turnaround on that message.
Just as importantly I don't think it should be overlooked that Trump is still in office literally right now, and is still doing everything he possibly can to fight the election result and discredit Biden's administration before it even begins. Biden coming out before he's even sworn into office and promising that the DOJ is going to drop the hammer on Trump just gives him all the ammo in the world to throw out to the 49% in the country that Biden's trying to orchestrate a coup by stealing the election and enacting a personal grudge and all other manner of projection. Yes you can say that the people who buy the shit he flings don't really matter since they lost, but the likelihood of Trump using those comments to incite further (violent) protest starts at "high" and only goes up from there. I don't hate the notion of him calling for unity for the time being and taking some time for the election furor to run its course and the national temperature to lower a bit before investigation of the previous administration's wrongdoings would even be effective or tolerated on a national level. There's little to be gained right now from throwing more gasoline onto a situation that Trump is still gonna be actively throwing matches at for a while, when he's still in the position to do so.