Edit: but feel free to explain to me how "Ethereum's gain (against $USD) is because people are buying BTC, not Ether" works

Certainly.
Ever taken a train ride ? Well if the train's travelling at 80 MPH and you take a stroll to the buffet car for a coffee, you'll actually be doing around 82MPH - 2MPH faster than the train.
There's a reason that every exchange expresses all alt-coin values in one denomination only and thats because one coin serves as the front door for this entire market.
(And it ain't no Lego-Coin)
