How do I compute the required bandwidth for a given bitrate of digital TV signal?
They use 704 for the horizontal x 480 vertical, which is typical for MPEG-2. That works out to 162Mbit/s. That's their mistake - they're putting the MPEG cart before the horse in computing the raw pixel rate for CCIR656-type format. They should have either computed it based on 720 or explained why they used 704.
Speaking of which... why 704?
Part of the reason 'only' 704 is used is to allow for the left and right edges to be smoothly transitioned from blank to active and back to blank again. This reduces ringing and artifacts at the edges when processing and encoding the signal. Many production houses still use 704 as the ‘safe’ area, especially in DVD authoring (hard to imagine that anyone cares much anymore about that dying format, but here we are) and use fade-up/fade-down at the edges.
It's also possible to synthetically make smoother left/right edges outside of the official CCIR 720-pixel active area and encode the whole 720 - for example, by replicating the leftmost and rightmost pixels. That takes more work. In 1995-ish that 'more work' mattered, not so much anymore.
Another part of the reason for 704 is to make the number of pixels a mod-16 multiple to be compatible with 352x480. Again, relates to filters and again, in 1995-ish that mattered, not so much anymore. (Video CD, anyone?)
And now... bandwidth
For the bandwidth part they're assuming 2 bits/Hz symbol encoding. This is the theoretical maximum using 2-level serial encoding, such as that used on Serial Digital Interface (SDI.) This is an extremely simplistic assumption, given that narrower bandwidth could be used with more bits per symbol.
And it is indeed the case with 8-VSB for ATSC and QAM-64/256 used for cable, which can not only carry compressed SD but also HD, and even carry multiple streams in the same 6MHz legacy channel.