An update on this.
I was told that allowing both was intentional because folks wanted to have Plain Text groups but also wanted to include the "Reply To Sender" link in the footer which is there on the HTML footers but not the Plain Text footers, so by allowing both settings checked one could have a group which converts incoming messages to plain text, but sends it out as HTML, so the HTML footers get used.
In this case I guess we should add some NOTE: or TIP: about it in the owner manual. I still believe the blurb(s) should be reworded.
I should start a new topic then because that workaround is not working, when both are checked one gets the same real plain text message with the plain text footers they get if Plain Text is checked only by itself.