> Is BTC a security?
Probably not in the traditional sense. The difference Bitcoin has to most cryptocurrencies is that it was specifically designed with no "premined" blocks and no preordained rewards. In contrast, a lot of projects sold tokens through an "ICO" with a promise of a future project developed against it. ICOs are basically securities, because you're buying the token without actually any guarantee that the project delivers on what it needs to deliver on for the value to change.
In contrast, again, Bitcoin was just offered as a cryptocurrency, from the start. The first block encoded a current event to distinguish the fact that there aren't any rewards already allocated to creators. It was also launched with the complete product available from the start (sans network upgrades).
Note: I'm not saying that Bitcoin is good or anything. I'm just distinguishing between it and most other tokens. Bitcoin has governance systems in place and other systems that make it "more security like" but clearly the SEC chair doesn't seem to believe it crosses that line.