But the ETF isn't going to distinguish between day-traders and long-term holders. In particular, Gemini's price was also in the $1300s before the news, and dropped significantly. If Gemini was giving the hodlers' price, that would be different.
If a handful of people trying to spike the price and trade off the news can cause BTC-USD to be 30% more than what it "should" be (i.e., what the price would be in a high-information, high-liquidity, low-shenanigans market), then the SEC is absolutely correct to say that the ETF is too wild to be approvable at present.
For comparison, on June 24 (Brexit), GBP-USD fell from about $1.50 to about $1.35, a 10% drop, and that was (IMO) much more significant news about the long-term future of the pound.