Was Bum's uncle an hallucination?
To be honest, when I was reading that part and when the uncle left I was thinking ‘Wow, that was unnecessary!’. Bum hasn’t seen his uncle in so long and it’s gonna take awhile for the police to actually be able to find Yoon Bum’s relatives (guardians?) and call them (why would they even do that? Bum is not underage. and they told him he was free to leave. The uncle showed up as soon as Lee left. Who even called him?). But suddenly he shows up out of nowhere, he steps into a stranger’s house without questioning anything just to assault Bum and then leave? I don’t know, seemed really strange and unnecessary to me. So it makes sense that it was just Bum hallucinating. Also I’ve never seen Bum so bold before. It might be all in his head. It might all just be the things he actually wants to be able to do.
Also it reminds me of another parallel. Remember when he hallucinated once about stabbing Sangwoo?
Another reason why it might be hallucination:
Hopefully this is not another art mistake but as you can see the blood disappears when Bum wakes up and he is wearing his shorts again. So all evidences so far prove that it was just hallucination.