If someone has non-alcoholic drinks they think are alcoholic and then they proceed to get "wasted" and make an ass out of themselves, is that a legit placebo effect happening?

Long ago I attended a lecture by a researcher who studied exactly this phenomena. As I recall it, they found that context is very important. If you are in a setting you associate with drinking and you drink something that replicates important elements of the experience, the chances of you subjectively feeling intoxicated increase substantially. People would be shocked to find out that they had been drinking cocktails that only had alcohol on the rim or non-alcoholic beer at the end of the experiment because they had felt it so strongly. Almost everyone they tested with replication of a real drinking experience maximized felt like they had drank alcohol to some extent and you could see their behavior change in very obvious ways as if they had consumed alcohol. The less they replicated the experience the less people felt it, so a nonalcoholic cocktail with alcohol around the rim you smell as you drink was more impactful than the same drink without the smell of alcohol on the rim, and whether the experiment was done in a bar like environment or a lab like environment made a significant difference.

It can be either.

It is possible to experience inebriation without intoxication via placebo effect. I've used that a few times to help people get to sleep or reduce pain perception, and fairly universally they'll also say they feel high.

But, people will also fake it out of perceived social onus. If everyone is acting drunk, a good proportion of people will do so even if drinking water and knowing they aren't drunk. When they think they're supposed to be, and don't feel it, the urge to behave as part of the group can kick in, and they'll act like what they think they're supposed to.

Myself, I would do that back as a teenager. Had no desire to get drunk, but didn't want to bring a party down. So I'd carry the same drink all night, sipping on it and act tipsy enough that everyone was happy.

That changed after I got drunk the first of two and a half or three times. After that I'd just smile and say I was good instead of faking anything.

So it's difficult to tell the difference from the outside. A lot of the perception of being drunk/high depends on familiarity with what the person thinks they're consuming. Someone that drinks plenty is likely to experience something closer to actually being drunk than someone with no experience. So to the outside observer, an inexperienced person and someone faking might well be indistinguishable

Interesting note on this point.

The kinds of behaviours we see when people are intoxicated is very cultural dependent.

The loud drunk, for example, is not something you see in every culture!

And of course, settings within a culture vary and essentially form subcultures in themselves

Yeah, that can 100% happen. You can even get people drunk through hypnosis. They know there's no alcohol involved but they'll still feel it. I've done it as a party trick plenty of times.