That's a sloppy statement. You haven't defined what standard. Clearly, everything is held to "a standard"; making that an entirely empty claim.
> When the rhetor introduces errors in the artifact, the rhetor's ethos with the audience is diminished.
A "rhetor" is a teacher of rhetoric. This is not the correct word in this case. In this case, the correct word is the more general "author", since the post was not teaching rhetoric. Further, the entire point of "ethos" in rhetoric is that we shouldn't be so lazy as to allow minor issues cloud our judgement.
> The more fundamental the error (getting a basic equation wrong I guess?) the more trust you lose with a knowledgeable audience.
Quite the opposite. A knowledgeable audience can decide whether to trust something based on the actual content, rather than minor surface issues. Only a lazy or uninformed audience need get distracted by typos.