The article says they reversed some transactions up to 22 blocks later, which is 220 minutes, or 3 hours 40 minutes. So they would only need 51% for that time, but they didn't just add 51% of hashing power or that would be noticeable on the charts. From the looks of it [0] the hashrate has been falling slowly since BTG was created, so the miner may have just become the one with 51% by virtue of keeping mining with the same setup and then watching others stop mining, increasing his share of the network with no extra effort on their behalf. I would assume this miner also realises that BTG is dead/dying and is trying to extract as much as possible from it before they move their equipment to a new chain. If this attack method proves sufficiently profitable perhaps the miner will look at dying chains he can easily overpower instead of trying to mine a more profitable chain with a higher hash rate.

[0] https://bitinfocharts.com/comparison/bitcoin%20gold-hashrate...