The EMS is simply the Mangekyo Sharingan without blindness. If swapping eyes meant you'd get the powers of the former user along with your current powers then that would be an outside power source. Since Sasuke never gained Tsukuyomi or any new powers from Itachi, along with Madara concerning Izuna, it's safe to say that it isn't an outside power source.
The Sharingan is their Kekkei Genkai, it's their own power from their DNA. Swapping eyes with someone especially when it doesn't give new abilities is not gaining an outside power because no powers are added.
An outside power is something that you rely on to give you strength that's usually sentient or does not belong to you, as in the Curse-mark and the Nine-Tails. The Sharingan is not an outside power if wielded by an Uchiha and the EMS is simply a negation of blindness. The problem is that fans of Naruto don't like the odds when SM Naruto is against EMS Sasuke but that's not my problem, Naruto could still win.