Update 1/7 10 am PT: Jeff Ross followed up his Tweet thread by appearing on Twisted Metal director David Jaffe's livestream to explain how the 8 million sales figure for Days Gone might not be entirely accurate, or at the very least, official.
As reported by PushSquare, Ross says the source of the sales numbers was from a Trophy data tracking site called Gamestat. During the conversation, Ross revealed that when he was at Bend Studios and saw Days Gone sold 5 million copies, the Trophy data was at 5.8 million. And since he no longer has access to Sony's official figures since leaving the company, Ross based the 8 million sales figures from trophy trackers and the "telemetry data."
"I think [Gamestat] still has the data archived," Ross said according to GameInformer. "Based off the 8 million mark, it matched our telemetry and so I'm like, 'good enough.' So maybe it was 7.7 [million]-ish, maybe it was 8.2 [million]-ish, I don't know."
So while Ross has previously found trophy data to be ballpark figures for sales, they are not wholly accurate compared to Sony's official 8 million sales announcement for Ghost of Tsushima.
Original Story: Days Gone director Jeff Ross says that the game outsold Ghost of Tsushima, but that it was still treated like a disappointment by studio management.