RTFA (and Wikipedia) reveals that the discovery of the galaxies is actually one year old.
What was discovered is that the mass of the brighest one found (1% light of milky way) is the same as the milky way (even if the nature summary talks about weight, tststs). The way they measured it is interesting:
The more massive a galaxy is, the faster its stars move relative to one another. These motions broaden the spectral line through Doppler shifts, [...]. By combining six nights of data, the astronomers found that the stars’ typical movements relative to one another clocked in at 47 kilometres per second.
What I would be interested in is how one is sure that one didn't simply misjudged the distance of the galaxy. If it was 10 times as far away as thought, it would also appear only 1% as bright as expected.