bitrate != resolution.
1080p (1920x1080) is a resolution it defines the number of pixels in an image
bitrate is basically how much bandwidth the camera takes and lots of variables/settings effect this. Typically this is given in Mbps.
Generally 30 FPS (frames per second) is overkill. FPS also is not bitrate as parts of that article might make you think.
If you set a camera to the absolute highest quality settings 30+fps, maximum bitrate, etc you're going to end up wasting a lot of storage space. An NVR's total incoming bit rate is arguably one of the most important specs, your article is only advocating that you estimate 5Mbps per camera (which is reasonable but not precise despite all the other nonsense in that article).
Take a look at the forum and NVRs that have been recommended recently. It's always better of have some headroom eg. you've got 8 cameras buy an NVR rated for 16 cameras.
Some cameras are higher resolution than 1080p (2MP), in the 4-8MP range and have even higher bitrates.