Then the entire movie is a lie and this is not Joker. You can't have one without the other.
They seemed to use the name to guarantee an audience. If the movie was name Arthur, and there was no Wayne/Gotham reference, would anyone have connected this with the actual Joker? No.
The Batman Joker is a criminal mastermind that can go toe to toe with batman over and over. That flat out is not Arthur. To remotely connect the two, you'd need to see an Arthur that is a formidable threat to even the police. That's not there. You can't watch that and say the police can't take that guy down. Maybe he can rise to that level after 3 more flicks, but they're not going to do that.
If they want to say that's the origin story it only works as a portion of the origin of the Joker we actually have known for decades. I don't think you learn mastermind abilities in your mid/late 30's, (or even older) as Arthur is. Nothing shown indicates he has the intelligence to get there.
That's the big flaw. To my knowledge this was supposedly an origin, but it wasn't going to be an extended production, just a one off. And maybe they'd do others. 89 Batman gave a much more believable origin story of the Joker (chemical bath aside)....Gangster/crook high up in an organization who didn't like the head honcho, snapped, and took over the whole thing and made everyone scared to death of him.
Do you see Arthur running a large organization? Getting people to follow him? plotting Machiavellian schemes? I don't anyone watching that and making those assumptions? Arthur isn't really that smart, a criminal or thug, he's just a mental patient who has snapped. I just don't see any connection to the character and criminal mastermind of the Joker we know. I couldn't even fathom how he gets there.