It is a reboot, but also a sequel in the chronological sense.
The doom guy in 2016 is the original doomguy torn from his slumber by alternate reality humans. Plus I recall them saying that its not a Sequel or a Reboot. At one point during the game, one of the … But at one point some demons …

You play as the marine that survived all that stuff in Doom 1 and Doom2. I think the RedLetterMedia guys put it best, it's a "de-boot".
Doom 2016 aimed to capture and modernize the spirit of old Doom's fast gameplay rather than get away from it, and it also has enough connections narratively that go just a bit deeper than being mere winks and nods that I feel safe calling it a sequel. That or Doom 2016 takes place inbetween Doom 1 and 2, directly after his ordeal on the Mars Moons. 4 minutes ago.

May 17, 2016 @ 1:50pm. Yes. A sequel would mean its a direct continuation. You foiled Hell's most recent plan, and Hayden locks you away because otherwise you'd prevent him from taking the Crucible and destroy humanity's future. Doom 4's ending was pretty complete. It took them quite a bloody while and he slaughtered so many he put fear into the Demons heart. Doom 2016 had its goofy moments, but Eternal embraces that side of its predecessor and runs away with it. The reason the Doom guy was chained up was because he was trapped by the Demons. There's nothing "to be continued" about that - it just keeps room for a sequel, rather than shoving the potential for a sequel in your face.