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?