Common usage is that "guys" is only truly gender-neutral with the "you" before it.

But yeah, that waiter did overreact. If you think you insulted someone, then you should be going out of your way to give them extra service, not ignoring them!

I make an effort to say "you all" instead, which people hear as "y'all". Oh well, better to sound dirty south than dirty sexist.

As you note, the sexist convention is reinforced mostly in the waiter's effort to apologize for (what he believed was his) mistake.

But I think another power-relation structures this encounter: that of the service employee in the workplace. The waiter's livelihood depends on achieving good vibes with customers (leading to tips). I can say that when I was a waiter, not all women seemed to think that "guys" was gender neutral. Occasionally I was corrected.

It's interesting to think back on that job. The trick was to follow the customer's lead. If women wanted to flirt, I'd flirt. If men wanted to flirt, I tried to project comfort with the gesture. If someone told a really stupid joke, I acknowledged their sense of humor. If people wanted to be left the fuck alone, then I'd try to do that. The last thing you want to do is insult someone. Unfortunately, everyone has different ideas about appropriate speech.

The worst was when you fucked up on the first approach, which is what the waiter believed he did. Ideally, he shouldn't even have acknowledged (what he believed was his) mistake, modified his speech, and moved on. This whole blog post stems from the awkward apology, but SOMETIMES that apology is expected. You know what I mean, dude?

"You guys" is definitely on its way to gender-neutrality, but maybe the last place that that neutrality will take root is in the alienated encounter of a waiter with a table of strangers.