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I'm trying to understand this so I don't know a lot of what's going on here, but why is the fundamental half the bit rate instead of the bit rate?

Imagine a signal that alternates every second (e.g. 10101010). This has a frequency of 1/2 Hz, not 1 Hz, because it repeats every 2 seconds.

I'm not 100% sure, but I'm assuming because of Double Data Rate transfers: https://en.wikipedia.org/wiki/Double_data_rate