Yes, the manual vs. spec is splitting hairs, but one point really was that you can get the spec now for no cost and start learning it at your leisure.
The other point was that it won't even be binding for several years.
@JoshPlum:
"...my office wants to get a head start on implementing the new code before engineers are required by law to have it."
The 15th Edition (or any preceding edition) of the Steel Construction Manual is not required by any law. It's simply a design aid.
The Specification for Structural...