It's not moving fast enough to not get pulled in though.
But, it is. (Due to how orbital mechanics work).
It's on a heliocentric orbit very similar to the one Earth has. Plenty of these objects exist for every planet (jupiter especially).
The chances of this happening obvious are low, but it can happen, and has probably happened millions of times in the earths history.
This object is also passing 280,000 miles away from the earth, which is about 15% over the distance from here to the moon. At that distance (and the relative speed it would be traveling), it might be thrown off by a couple of degrees in orbit, but since it follows a similar heliocentric path it'll be back eventually (in 2017).
Whether or not it's an old spacecraft from the races or just a big rock, that's to be found out. The chances of it being alien in origin however are slim to none. The discoverer only brought that up as a possibility to disprove it.