In civilization V all unique abilities was tied to the leader or atleast listed on the leader's page on the civiliopedia.
This is because it was built on the Civ4 engine, which tied abilities to leaders. This does not disprove my theory.
In civ VI, yes the mention civ abilities and leader abilities but that don't necessarily mean that they are going to make several leaders for one civ, it could be to give the civ an ability that fit well with the leader.
Then they would just say "due to [leader]'s reign, the civilization accomplished [X]. This is represented with the [trait] part of this civilization's unique ability, [ability]"
If they create new civs they are going to create new leaders as well so they could say civilizations and leaders just to make it sound like you get more then you get.
Just to check, I went back and looked at the descriptions for Civ5 expansion packs.
"The expansion will deliver 9 new civilizations, such as Carthage, Netherlands, the Celts, and the Maya (it is 2012, after all), along with their unique traits, units and buildings. The expansion also includes 9 new leaders including William I, Prince of Orange, Boudicca and Pacal the Great."
So you are correct that they have done this before. However, this also points out that they missed the comma after Boudicca, so their grammar is not really something to go by.
Most importantly, we have no civs that fit E1 and B5 alphabetically that we could imagine being in the game. An extra leader seems more likely. Perhaps even more likely than that, these extra leaders are just meant for scenarios or brainstorming.