Yes.  I stated that passing an already sufficiently secure private key through SHA-256 wouldn't make it any more secure than it was to start with, however I failed to point out (as you have) that passing an insufficiently secure seed through SHA-256 also wouldn't make it significantly more secure than it was to start with.

SHA-256 is a reasonable way to take something that already has more than 256 bits of entropy and convert it into a 256 bit number that can potentially be used as a private key.