I'm puzzled a bit by the lack of discussion of this distinction in other accounting & tax publications. I've read several articles by law firms purporting to "explain in detail" how this all works, but every single one of them leaves the reader thinking Sep 30 is a "drop dead" date. (Perhaps they are trying to create a sense of urgency, which also might translate into premium billing for the work?)
Yet, at the same time the Journal of Accountancy is certainly a reliable source so I have no reason to doubt their analysis. The only thing I can surmise is how averse many people are to "penalties" in the generic sense. For example, a FTP penalty is peanuts when compared to a FTF penalty, but I sometimes question whether a particular commentator understands the difference since they seem to treat all penalties as the same.
Whatever the case, if a taxpayer owes $10K and can't make the Sep 30 deadline but could be ready by Oct 31, they pay a $500 FTF penalty vs $2,500. And so on, with the penalty increasing by $500 with each succeeding month until the 5th month. (I'm not taking into account the interest and the relatively tiny FTP penalty, since they remain the same under any scenario of filing and payment) As you said, Lion, this is huge. Especially at this late date.