When the rhetor introduces errors in the artifact, the rhetor's ethos with the audience is diminished.
The more fundamental the error (getting a basic equation wrong I guess?) the more trust you lose with a knowledgeable audience. If the author doesn't see that a fundamental issue was introduced, they may not have been expert enough to not introduce additional errors; the reader must spend more time double-checking the components of the argument rather than thinking about the argument itself.
If someone comes to this article as a novice in the topic and stores the error as a fact, they may end up at least confused when approaching it again in the future. HN tends to have an audience representing deep knowledge in many fields, who end up providing a thorough and varied set of quality filters. These quality filters are also really helpful to the novice who may otherwise miss the typo.