I'm not well versed on how an engine affects the game as a whole. However I don't believe it's merely coincidental.
Oh, there's nothing coincidental about it. The art style was a deliberate choice on the part of ZOS. And I will explain why
Stylized art ages far better than photorealistic art. It won't look any more or less stylized ten years down the road than it does now. However, photorealistic art will show its age within just a couple of years, as newer technology for photorealistic rendering is applied to new products.
For example, When Doom III was revealed, people were amazed by its graphics. But look at it now and it might as well be a steaming pile of demon crap. Single-player or stand-alone games can get away with photorealism, because there's always going to be a next game waiting in the wings that will maintain the cutting edge on graphics.
But an MMO must stand as-is for years. MMO developers do not have the luxury of overhauling their graphics at regular intervals. Whatever they go with has to look as good as it can for as long as it can...
And a lot of times, stylized graphics can actually improve as new shader and texture mapping technology gets implemented. If you look at TOR on a machine running with low-end specs and compare it to how it looks on a machine with high-end specs, there is a marked improvement. Higher resolution, smoother shading, etc. And if the base art is decent enough that it looks acceptable on low-end, then you've got wider audience at the onset.
I'm not too worried about TESO's graphics. It is the gameplay which will be the make-or-break factor for me.