You do realize you catch Noibat at level 44 right? (not counting Safari Zones) That's only 4 levels if you catch one in Victory Road its past the level it evolves.
And if you never use Pokemon Amie, your Eevee will never evolve into Sylveon.
If you didn't cheat and looked it up or spend hours testing every theory, your Eevee would take longer to evolve. Secondly if you didn't know Eevee needed a fairy type......
Don't let hindsight make it seem like Eevee evolving into Sylveon is easier than Noibat.
Noibat will always start to evolve at level 48 regardless of the circumstances (not counting everstones, obviously), so no Noibat is easier to evolve than Eevee at least into Sylveon.
Eevee is more likely to evolve into Espeon/Umbreon than Sylveon.
Edit: Just because Eevee could become Sylveon at a much earlier level, doesn't mean its easier. Pancham starts to evolve at 32, doesn't mean it won't take someone until level 100 to realize it needs a dark type as well to evolve.
The only way getting a Noivern is easier than getting a Noibat is if :
1)You caught a high level Noibat
2)You got traded a high level Noibat
3)Someone never bothered to check on how to get a Sylveon which if he actually wanted to get,would have checked in the first place.
So yeah,in that extremely specific scenario it is a lot harder to get a Sylveon than a Noivern.So your right,in that extremely specific scenario.In general or a large majority would find it is so much more easier to get a Sylveon than a Noivern unless your saying that people in general or the majority of players don't check to see how they are suppose to evolve a pokemon.
But let's look at the scenario in the anime :
1) Ash's Noibat was caught as a baby and has not been used much meaning it does not have much experience
2) Serena's Eevee already has the friendship requirement(both normally and the pokepuff version) checked.All it needs is a fairy type move and a battle to evolve which it might be getting here.
In this scenario it is so much more easier to get a Sylveon than a Noivern .