How can one provide feedback about your immediate supervisor without coming off as a backstabber?

Context: my immediate supervisor provides minimal guidance in my work and is uninterested in pushing for things/initiatives that management wants him to drive. For the former, he sees it as giving me autonomy to prioritize my own work.

Recently, his boss had a short informal one to one chat with me and asked if I am being provided with enough guidance. Prior to this, they had many clashes, both direct and indirect, talking about his lack of output, which my immediate supervisor does not acknowledge. His boss knows I am struggling and stated that it is not fair that so many things are loaded onto me.

When he asked me this, I was unable to give a specific reply and only mustered a soft “got la…” and weak smile. I didn’t want to state that my immediate supervisor is actually not really driving things (which he already knows) and start stating examples of when I was left to work on something when everyone has gone home (which he also already knows).

What makes this slightly complex is that my supervisor is actually quite a nice person outside of work. We are on friendly terms.

The boat has sailed, but I know there should be a professional way of approaching this question without coming off as a backstabber or turning it into a complaint session. Or in this case, leaving things unsaid. Help out a sis here, any tips? 🙂

Also… Why does he want to hear from me when he clearly already sees and knows what’s going on?