If being a barista was completely superfluous to running a coffee shop, why would companies like Starbucks even bother hiring them then lmao.

It's pretty obvious these patsoc losers are trying to abuse Marx's concept of productive and unproductive labor. But productive labor as understood by Marxists within a capitalist society just means labor that contributes to the surplus value of the particular commodity being sold. A commodity is a good or service that is for sale on the market, meaning the production of a commodity is not just the physical manufacturing of a good but the distribution of the good in the market as well. To use an often quoted example, someone who carves a wooden chair for personal use hasn't created a commodity because the chair is only for private use. It's only when they place the chair on the market for sale, be it a physical store or an online store like Amazon, that the chair becomes a commodity.

From this lens, baristas obviously perform productive labor because not only do they produce coffee drinks from various ingredients, where the exchange value of the final product is greater than the sum of the exchange value of the constituent ingredients, but the baristas are also the final step in distribution of the drink within the market by virtue of directly interacting with customers. Even ignoring the part where baristas perform socially necessary labor through the production of drinks, without baristas, the drinks would all be sitting in a fridge outside the reach of customers. Within the framework of capitalist property rights, customers can't just walk into the back of the shop and take it out of the fridge, meaning those drinks, by being barred from customers and the market, aren't commodities yet.

Within a business firm, actual unproductive labor from the prospective of capitalists is HR or the accounting department or internal counsel because none of them directly produce surplus value of the commodity. It doesn't mean they aren't necessary or useful to the business. Even if they don't directly produce surplus value, Starbuck's accountants might save the company millions of dollars by exploiting various bullshit tax loopholes and Starbuck's lawyers might save the company millions of dollars for successfully defending the company in court.