TL;DR
Here is the meaning of the sentence: "Broadcast is seen as a medium. Yet the Internet is seen even more so, and that is because the Internet is not just an environment, it's also a tool or a highway."
You can read:
"that it is seen as a medium much more so than is broadcast" as:
"that the Internet is seen as a medium, and it's seen so much more than broadcast is seen so".
The following is step-by-step transformations showing that the sentence is grammatical.
(By reading these steps carefully, you can understand the meaning of "that it is seen as a medium much more so than is broadcast" and the structure of the sentence. It's a bit tedious, but I hope that you'll be able to apply the idea to other long and complicated sentences you may find in your reading.)
Eliminated less-important parts:
However, it is precisely because, in an everyday sense, the Internet is seen as a tool, or as a vessel/conduit ‘highway’ (see the discussion below of Meyrowitz’s three metaphors of media), rather than an environment, that it is seen as a medium much more so than is broadcast.
Replaced noun-phrases with symbols.
It is precisely because the Internet is seen as T or as H rather than E,
that it is seen as M much more so than is B.
Decomposed the clauses:
1a) The Internet is seen as M much more so than is B.
1b) It's so (1a), precisely because the Internet is seen as T or as H rather than E.
Unwound 1a):
2a) The Internet is seen as M. -- It's so (i.e., seen as M) much more than B is so.
2b) It's so (2a), precisely because the Internet is seen as T or as H rather than E.
Unwound information:
3a) The Internet is seen as a tool (T) or a highway (H) rather than an environment (E).
3b) (implied) Both broadcast and the Internet are seen as a medium (M).
3c) It's precisely because of (3a) that...
3d) The Internet is seen as a medium much more than broadcast is seen as a medium.
And that's all about it (about the structure of the sentence)!
The rest is to find out what the keywords (broadcast, medium, environment*) mean in this context, in order to understand the meaning of the sentence completely.
Here are the key important ideas:
Broadcast (or broadcasting) - the distribution of audio/video content to the audience via mass communication medium. TV is the representative of the "broadcast" medium.
Medium (in the context of "media studies") - A medium is anything that mediates our interaction with the world or other humans.
Environment (in the context of "media ecology") - the media of communication that is surrounding us. "The word ecology implies the study of environments: their structure, content, and impact on people." (Wikipedia)
With the understanding of the keywords and the structure of the sentence, I hope that you can understand the structure and the meaning of the sentence. Here is my paraphrase of the sentence:
"Broadcast is seen as a medium. Yet the Internet is seen even more so, and that is because the Internet is not just an environment, it's also a tool or a highway."