Did you happen to notice in the old thread that Jamie also wrote “the period”? Your question is a valid one — and so was my first answer.
I have a suggestion. Take a look at the two links below. Can you draw any conclusions about saying the comma vs a comma based on the Google search results? (Please ignore results where the author obviously didn’t know how to spell “coma”.)
Sometimes we use “the” when we are describing something generically: “The elephant may become extinct.” “The perch is one of the most popular fish to eat in the Midwest.”
Obviously the first sentence doesn’t mean that just one elephant may become extinct, but all of them. People in the midwestern states are not fighting over the same individual perch to eat, but like to eat any fish of the perch variety.
Because I was talking about a general principle as to where to put a comma, I said “the comma”. It describes the punctuation mark in general, rather than just one specific comma that one person writes on one particular occasion. However, if my mind had been thinking of it differently, I could just as easily and correctly have said “a comma”.