Wow, bad one. While we can all understand how it happened, it's quite unacceptable at this level. The offside was so bad that I'm actually disappointed the referee didn't pick it up - even without communications he should have spotted it and made his own decision, but with the comms surely he would have asked the question? I seriously have no idea what the purpose of those things are, they don't seem to help decisions at all.
It's very rare that I'd expect the referee to overrule the AR on offside positioning, given he needs to have that trust and he's not looking for it - but I can't believe the referee missed this one as well.
But, for a more constructive discussion, here we have a complete, inarguable offside. If a goal was scored, I'd even like the far AR or 4th official to have a chat to the referee here and let him know what he's seen. It's bad enough to justify that sort of assistance.