Must an independent clause follow a colon (if it's not a list)?

Question

I wrote this sentence:

The unregulated internet we are accustomed to won out over another vision for the World Wide Web: a tightly controlled, "government-sponsored broadband network that would have delivered video from TV stations and other approved content"—another FCC-regulated medium like radio or television.

A TA subtracted a point, explaining:

Unless you are listing two or more items, an independent clause should follow a colon.

I know an independent clause must precede the colon, but I can't find anything to back up the TA's explanation. The rule of thumb, according to a couple of popular grammar sites, is that if the colon can be replaced by the word "namely", it's usually OK. Such seems to be the case in my sentence above.

Is my sentence grammatically incorrect?

Answer 1

Point your TA in the direction of the UK’s University of Sussex, which publishes a Guide to Punctuation by its late Professor of Linguistics, R L Trask. Of the colon, he wrote:

The colon is used to indicate that what follows it is an explanation or elaboration of what precedes it. That is, having introduced some topic in more general terms, you can use a colon and go on to explain that same topic in more specific terms.

There’ s nothing there about the need to follow a colon with an independent clause, and many of his examples don’t have one.