In this case it's not actually that big a result, it's an incremental improvement on a series of previous results. Cryptographer Orr Dunkelman describes it better than I ever could, he's one of the people who produced one of the previous results:

  The main result in this paper is improving the Derbez, Foque, Jean attack from EUROCRYPT 2013, which is an improvement of our attack from CRYPTO 2010, which is an improvement of the Demirci-Selcuk attack, which is the improvement of the Gilbert-Minier collision attack against 7-round attack [...]

  To save everybody's time, the [DFJ13] attack is on 7-round AES. The new result is also on 7-round AES, "eroding" the security margin of 7-round AES by about 8 bits of security [...] While this is the first improvement in attacking 7-round AES in the last decade, if you were not worried by the series of papers that reduced the security of 5-round AES from 2^32 to 2^16, or the somewhat improved attacks on 6-round AES, then you should not really worry now to start a procedure for changing 10-round AES (for 128-bit key) for something else, when there are no attacks on 8-round AES-128.

So someone threw a clanker at a series of previous results and told it to find improvements. Since it's ingested every piece of crypto research ever and can draw on all of them instantly, it managed to tweak the previous work a bit to improve the attack slightly... on a version of AES deliberately weakened to make attacks easier, a standard procedure for any iterated crypto algorithm where you see how many rounds you can get into it before your attack stalls. So it's a bit like saying you knocked out Mike Tyson's brother's cousin's uncle's sister's nephew in four rounds instead of five.