Through out the years all the previous canon sources have quoted the edict as "ban on sorcery" ( Index Astartes et al ), NOT Psykers in general. This was logical and consistent with the way the Imperial forces have worked post-Heresy. That is why there are Librarians running around ten thousand years after the Siege of Terra for crying out loud!
Of course the previous background (presenting the Heresy as a historical event that took place ten millennia ago) conforms to the setting as it exists 'now'... but history in the Imperium is revisionist, and the records of a mythic time of galaxy-wide warfare during the early days of the Imperium are not inherently going to be accurate, particularly if there are people adjusting, adding and removing information to suit the politics of the time.
The Imperium of the 41st Millennium is a twisted mockery of everything the Emperor ever intended for humanity... it it really any surprise that the 'historical facts' told by the men and women of that Imperium are only superficially similar to what actually happened.
But for some incomprehensible reason this novel now suddenly made a break from this established fact. I call BS. I see it as either a case of 1) poor editing, 2) lack of background checking, 3) "in-character variant view by Thousand Sons" or 4) intentional retcon. 1 and 2 are irritating possibilities but can be overlooked since mistakes can and do happen. 3 is palatable if we are given the "other side" of the story that corrects the error. 4 would be just plain stupid. It would mean that the most loyal of loyal, the Emperors own Astartes, have de facto defied His will ever since the Heresy. Yeah, that would really make sense.
The Heresy background for the last few years has deliberately included many elements that don't entirely gel with what many before had taken to be absolute truth; given that we don't know how this version of the story ends, except in the broadest terms (the Emperor confronts Horus on the Vengeful Spirit above Terra, Horus dies but the Emperor is mortally wounded in the process, etc), protesting that 'it doesn't fit with what we know' is a hollow argument, IMO... as we also don't know how the differences are reconciled. Given that the author of this particular novel has, in the past, also written earlier versions of Codex: Chaos Space Marines (3rd edition, 2nd Codex), Codex: Space Marines (4th edition) and the Warhammer 40,000 Rulebook (4th edition), I think it's a reasonable assumption that he knows more than a little about the 40k background in general and the history of the Astartes in particular.
Writing Credits for Fantasy Flight Games: Into the Storm, Edge of the Abyss, Battlefleet Koronus, Hostile Acquisitions, Black Crusade Core Rulebook, First Founding, The Jericho Reach, The Soul Reaver, Only War, The Navis Primer and Ark of Lost Souls