I wouldn't be surprised if it has done better on Steam than on any other single platform. We'll know when Ars publish their next article I guess.
1. I highly doubt these numbers even include Steam.
2. There is little chance it became the best selling since the PS2 gen due to being on Steam or that its the best selling version.
DBZ XV did exceedingly well in EU charts, NPD and even in Japan. Best selling SKU is most likely PS4.
I don't see the PC audience being notable going by Ars sales.
Naruto: Ultimate Ninja Storm Revolution did 650 000 in less than a month on PS360. Steam sales have it at 100k for around 4 months. UNS3 has shipped 2 million WW on PS360. These types of games have massive audiences on consoles especially Playstation.