That is like saying the 601 limousine was Trabant's flagship product. Sure, yes, you are technically correct but... no. The Nexus devices were less than half the price of actual flagship products.
Back when the Nexus came out, the top iPhones were selling for $599, if I recall correctly. So at $400 it was more than half, but that's splitting hairs because it was Google's flagship. By sales volume a bit player, but it occupied tremendous mindshare because of the brand.
I paid $400 (or more?) for what was billed from google as their premier nexus device (I just got whatever the lowest RAM was). I got it for a testing device. What was lauded was "doesn't come with carrier bloatware/apps". OK. Fine. It was a middling experience at best, but was presented as Google's 'best' at that time.
"Devices in the Nexus line were considered Google's flagship Android product"