The point is: historically there were times when there were more than 7 electors...
If by the 8th you mean Bohemia, that is somewhat legally unclear whether it may be called an official Electorate...
I suspect he's referring to some of the shenanigans during the 30 years war: the Palatinate electorate was revoked during the war and given to Bavaria (both were Wittelsbachs); but at the Peace of Westphalia, the Palatinate got the title back, so there were eight+ from then on. Brunswick/Hannover got the title in the late 17th century (don't recall exactly why), so there nine for most of the 18th century, until the two Wittelsbach electorates merged, via inheritance, bringing it back to eight in the late 18th century.
When titles were revoked, they were often given back during the peace deal (e.g., two more were revoked and reinstated during the War of Spanish Succession).
I think, technically, some were also added during the Napoleonic period, but these were during the death throes of the HRE, so they probably don't count.
Might be interesting to allow more, under certain conditions, maybe with some IA penalties. IIRC, there could be eight in previous versions of the game. But I'm not really sure it'd add a whole lot to the game. HRE representation is a bit of a fantasy, as it is, but an interesting and entertaining one; and one that works reasonably well. Not sure this is an area where greater historicity would enhance the game.
@OP - No, you can't ever grant more than seven, without changing the number of target electors in the defines.lua file. Some bookmarks probably do reflect the historical realities of eight or nine, though.