I think it's pretty okay for Biden to not directly support "defund the police."
First, it's deeply unpopular. As a general rule, it's not a great idea for a national candidate come out in support of something a big majority of voters do not want. I'm sensitive to the argument that a piece of policy could be unpopular but correct (we should do a thread of unpopular things that we think are the right thing to do, I have a few in mind), and also that a good leader can craft a message that can turn public opinion. But "defund the police" is a slogan, not a policy, and is one that is perfectly suited (as opposed to, say, healthcare) to drive change tailored to a local constituency.
I think there are real and fair questions to ask for those calling for abolition. Why do people in poor communities, majority minority communities, and high crime communities want *more* cops, not fewer, when polled? How do you respond to the data that suggest that more police results in less crime? What is the plan to replace all the services currently provided by the police, and if those services are better provided by other (non police) professionals (and they almost certainly are), couldn't we pursue and implement those policies directly, separately from abolition, at which point you'd have a much easier sell?
There isn't going to be one simple solution that works for everyone. And abolition/defunding in its various iterations would be a huge change! To riff off of the Sagan standard, extraordinary change requires extraordinary evidence. But with policing, I think we can get there. What Minneapolis is attempting to do is fantastic. And if it works, and we can show that cities and towns of varying sizes and demographics can successfully improve well-being and decrease harm with a huge reduction in what we currently consider policing, then I believe the national dialogue will start to change. But I don't think Joe Biden telling some reporter that he does or does not support "defunding the police" really helps or hurts that cause.