Bethesda: Doom will run 720p in both docked and undocked mode on Switch.

[Conina]

We don't know what the Bethesda spokesperson told him exactly.

Maybe they only told him that both the docked mode and the mobile mode have a 720p output (which doesn't say much about the native resolution or if the game uses dynamic resolutions and upscaling to 720p in handheld mode).

Digital Foundry compares the Switch version to the PC-version in 960x540: http://www.eurogamer.net/articles/digitalfoundry-2017-hands-on-with-doom-on-switch