Sure but I can make authoritative statements about something without knowing everything about it. A few statistics about wealth or something in communist countries would be enough to speak authoritatively about whatever those statistics are measuring. Also if there's something pretty much every expert agrees on from either side, then you can say that pretty authoritatively.
I'm not sure if according to Olavo we can reliably know about anything. Yet humanity still makes progress on complex topics. Maybe the bar for authoritative is just extremely high, but then it's probably too high to be useful.
It's not a question of knowing everything, but of knowing as far from nothing as possible. Take one of my hobbies for example. Wine. When you teach anything that interfaces with the infinitely complex real world, you need to engage in what is known as a pedagogical lie. The human brain needs to learn things from the outside in, memorizing broad categories and then refining them to specialize your knowledge of a topic.
In the beginning stages of this process, the knowledge you gain can cause you to make inaccurate conclusions with more confidence than if you were completely uninformed and approaching the topic with the proper level of epistemic humility. In wine circles, the people at this level are the dreaded wine snobs.
As an example, they have learned that Chardonnay is oaky, rich and heavy. This is a pedagogical lie. Many of Chardonnays are not - depending on slope exposure, latitude, altitude, climate, and élevage, the style can be rapier-thin, fresh, and highly acidic or dense, stony and closed in its youth, and many iterations in between. If I'm talking to somebody who knows nothing about wine yet has expressed a liking for racy, bright wine and ask them to try a Chablis, for instance, they have no preconceptions guiding them and will say 'okay, sure', then 'oh, wow, I really like that!'.
I've done the same to a wine snob, they will ask what grape it is, I say 'Chardonnay' and, faced with a false conception of what Chardonnay is, they will say 'oh, never mind, I don't like Chardonnay'. If they gained the surface-level understanding of wine a while ago and have developed a self-conception of being a 'wine person' around this, they will even be vehemently dismissive and refuse to consider the fact that they were wrong and might like this Chardonnay.
They know more than the complete neophyte, yet they are more wrong with more confidence - this is the meaning of Pope's paradox about knowledge. Skimming the surface is often more dangerous than never dipping a toe in when you're dealing with the real world - you develop incorrect, overly broad generalizations because you've only been exposed to that first level understanding, which is rife with pedagogical lies. You don't understand how much you don't know.
The same example holds for something like communism. Just as wine is as complex as the cultural legacies, geological landscapes, climate patterns, and biological evolution which has defined it, communism is an ideology which has gone through so many permutations and iterations in different cultures, places, and geopolitical and socioeconomic conditions that delving deeper into the subject quickly reveals the severe limitations of surface-level understanding that can be imparted in a brief lecture or debate.
And yet paying lawyers to argue is the most reliable system anyone has come up with to judge guilt vs. innocence.
Now for your example about lawyers. Lawyers are not interfacing with the real world. They are interfacing with a legal system, which is a human construct. They are arguing, for or against, the innocence or guilt of a party under law due to the nature of their actions. When it comes time for the trial to interface with reality, with the material world, a good lawyer does not do that. He brings in an expert to testify, because he recognizes the very fact that I pointed out - that sparring over the surface-level interpretations of very complex subject matter is not illuminating to anyone, and that to bring good information into the courtroom you need an interpreter of sorts who had put a lot of time and energy into discovering as much about the field in question as possible.
When it comes time to draft laws, the same thing happens - there is a recognition that there is a divorce between reality and the laws we make to govern how we interact with it, and that consultation is often necessary to conform the law to nature. And it is not the lawyer who determines the guilt or innocence, it is a jury, an uninformed jury. In a sense, the process of justice involves a fully uninformed jury watching superficially informed jurists argue while consulting deeply informed experts when the facts of the case touch on complex topics which are beyond the ken of anyone else in the room.