1. It has perfect logic to include 1 civilization with 2 leaders - to show and test the ability.
2. The Civilopedia clearly states some civilizations have multiple leaders.
3. Both Gorgo and Pericles are in civilopedia.
That's what majority of people here see and I'm pretty sure we're right.
1) There is absolutely no need for Firaxis to test and show the ability of multiple leaders. It was already a proven mechanic back in Civilization IV. Yes, that game came out 11 years but still... this is a twenty-five year old game franchise whose game iterations build off the previous ones so there is no real need to re-test an already used mechanic.
2) The wording implies it yes, but Firaxis hasn't come out and confirmed it. Until such time that they do, I will remain a skeptic of multiple leaders being a thing.
3) Again, while true that doesn't really mean anything. Ed said that the build was the most up to date in terms of development. They could still include DLC civilizations for the sole purpose of balance and testing and take them out of the vanilla launch version (only to be put back in as DLC). Considering the number of abilities tied to both leaders and their civilizations, Firaxis does need to test and make sure that everything is as balanced as possible. And with it known that there will be additional civilizations in the first four DLC, you can be sure they know which will be new and are testing them alongside the base game.
It makes no sense to include DLC in the civilopedia on release. Yes, it hasn't released but who would put future stuff in a couple of months before release then pull it out?
See Answer #3 above...