It works in the plural because it's commonly used in the plural. It doesn't work in the singular because it's not commonly used in the singular. That's my short answer.
My longer answer is that there is a continuing crisis (perhaps too serious a word, but we're all language enthusiasts, right?) in both second-person pronouns and gender-inclusive pronouns. While English no longer has distinct second-person pronouns to convey formality/politeness/social distance, we still use words (particularly forms of address) to convey those sociolinguistic matters. Like "sir" and "mam," to convey distance or formality. But those have become less common in many regions and many social realms. On the other hand, we see an increase in those that convey closeness or informality (and I'll confess North American-centeredness here), such as "man" and "dude" (which I've even heard adolescent women use to each other). This is part of the semantics of solidarity, which is used to bridge social distances, a bridge which can be built either way (from higher to lower or from lower to higher (roughly conceived)). It does indeed seem more and more common for those types of bridging projects to be approved unilaterally by the "lower;" (say subordinate or whatever you will if you don't like "lower"); i.e. people use casual terms of address even with those in a perceived superior position or across social distance. It's "semantics of solidarity." And please educate me about the situation in GB or elsewhere because I'm talking primarily about North America, where as a child I would never think of addressing my friends parents by their first names but now "Mr." and "Mrs." are becoming anomalous in this situation. Likewise other situations.
So the waiter's use of "guys" is wrapped up in those ideas. And I suspect that your question relates primarily to the semantics of solidarity, and secondarily to the singular vs plural.
On that point: we have many terms of address that are used exclusively in plural, such as "ladies and gentlemen." When have you heard "good evening, lady and gentleman"? And "guys."
Other terms of address can be used in both singular and plural, in the same situations, such as "dawg(s)," and "chap(s)."
And some terms of address are used in both plural and singular, but differently.
Uniformed officer to uniformed soldiers: "All right men! Fall in!"
That same officer, annoyed because that little puke Private Wilson wasn't listening, would not say to said puke: "I said fall in, man!" However, that same officer might, later in the officer's mess, talking with an equal, say "Man, that Private Wilson sure has his head up his ass."
And of course there are some terms of address that are only used in singular, such as "sir," "ma'am," and "bro."
Now let's return to the pizza restaurant. What is the acceptable plural, gender-inclusive second person pronoun that a waiter should choose when addressing a couple? It's a question that may not have confounded a similar waiter in our sexist past, when he would have simply addressed the man. But we've progressed [sic]. So what should he choose? I suspect that with people under a certain age, the prevailing semantics of solidarity allow "guys." But to many over a certain age (no, I'm not going to specify) the use of "guys" raises hackles. The alternative, for those in the grey area, is to simply avoid a term of address altogether and stick to the inclusive pronoun "you," a la "Good evening. Can I get you something to drink" [looking alternately at both people]. Or one may direct questions singularly to each half of the couple. Alternatively, many people where I am from will use "folks."
Of course, there are regional solutions as well, such as the American south's "y'all." Though a friend from Kansas informed me that "y'all" is singular and that "all y'all" is plural. Perhaps that's a distinction born as "y'all" migrated to parts of the Midwest.
A final note: you were in a pizza restaurant, drinking beer. If we define culture - in this blended global world - not by nation but by circumstance, your waiter was surely a conformist. And while he may not be a user at this site, he may have "posted" a question about your "guy" comment in the informal ELU that is every staff room, kitchen (restaurant or otherwise), and other gathering place of humans.