punctuation - Larry Trask seems to be violating rules about commas as mentioned in his own guide

I was reading Larry Trask's guide on how to use the comma. His node11.html mentions that we should not use a joining comma before any word other than and, or, but, while and yet.

But in node13.html, he seems to be making some dubious uses of comma violating the rules he has mentioned in his guide. For example,

"So, the bracketing commas shouldn't be there."

The comma used in that sentence is not a listing comma. It is not followed by and, or, but, while or yet. It is not a gapping comma either, and doesn't seem like a bracketing comma. Then why is the comma there?

Trask suggests that we fix such punctuation using semicolons. A comma should precede only the five connecting words he has mentioned: and, or, but, while and yet. For anything else, we should use a semicolon. But he doesn't do so in the next example. Why not?

"This is a good sentence, so you have now got the bracketing commas in the right places."