Hi William
The reason I quoted that rule was that I would expect the length of the DBE foot element is either less than or equal to the width of your element. However, in the case of a DBE mounted element (which, as you mentioned, has a length greater than the frontal width of your element) you can still make a legal flank contact by making a legal front corner to front corner contact - check out the rules for 'close combat'. A legal flank contact on a DEB mounted element still counts as 'front edge combat' so you still get the -1 tactical factor and quick kill.
Andrew