The operating mode described by the comment to which I was responding, that of CloudFlare performing all steps of SSL termination, is one operating mode available. CloudFlare offers at least one other operating mode in which they are not responsible for all aspects of SSL termination. In particular, they offer an operating mode in which they do not hold private keys. This is referred to as "Keyless SSL". Thus the concern voiced by the comment to which I was responding, that of CloudFlare stripping SSL, is but one available option rather than the only available option.
"Keyless SSL" is a bit of a misnomer. CloudFlare still holds the keys to encrypt and decrypt the content it is distributing and therefore sees everything (they can inject into your traffic as well). Even if they don't hold the private key, they can act in every way as if they did hold the private key.