Here's my experiences.
When I bought iPhones last year from a reseller (got from Qoo10), it is unactivated. The serial is seen on imei24 but not on apple checkcoverage. When the phone was unboxed a week or so after, connected to internet, placed apple account, from that day on it is visible and the one year warranty started counting. The iPhone8 Plus we even activated a month after the purchase date, and the counting only started then.
Recently I got an iPad from Apple Store in Orchard. I don't plan to open it that day, maybe only this weekend. However, the serial is already seen on apple checkcoverage, and the one year counting started on the purchase date. Even if the thing is still sealed in box.
Few weeks ago I had someone buy me a sealed iPhone SE from overseas, from PowerMac (Apple authorized reseller, the closest thing to an apple store because there is no apple store there). Until now, when I input the serial either on apple checkcoverage or imei24, it not found. However, when I input the IMEI in imei24, it appears the device is not activated.
So what I can surmise here, Apple has no tracking on reseller or distributor inventory units, so they depend only when the device registers on their servers. However the ones under Apple inventory themselves where they have full visibility, the activation comes on the purchase date, irregardless of when you really register to apple via the internet.