a : dissent or deviation from a dominant theory, opinion, or practice
b : an opinion, doctrine, or practice contrary to the truth or to generally accepted beliefs or standards
So, you can have a "scientific heresy", "political heresy", "social heresy", or even "40k list building heresy". It does not require acceptance of the Emperor as deity.
On the subject of the recent retcon, a number of people flinched and said they could "see that" BT might venerate those who had "touched the Emperor" (sanctioned psykers). I thought about agreeing, it kinda, sorta sounded reasonable, but something kept stopping me from doing so, even though I remember arguing that since the Emperor had sanctioned Navigators and Astropaths, the Black Templar would accept them, so it wasn't that big a deal, right? I found the answer in some of my recreational reading. It goes a bit, you have been warned.
"When the man had walked in Bannick had initially taken him to be some kind of priest from a mendicant order, for his appearance was modest and his manner that of an aesthete, and Bannick thought his enormous size another product of man's endless variety. It had not been until the man had been announced by the sergeant as the Brother Arnegis, Lord Steward to Castellan Thieme of the Black Templars..."
Setting is on a planet under attack by an Ork Waaagh! and the Black Templars are incidental to the storyline, being involved in the fighting on the opposite side of the world from the Baneblade of the title, part of two groups of Baneblade variants fighting on this side. The general on this side of the world is mentioned in passing to have petitioned Castellan Thieme for support after discovering that the Orks had a weirdboy Gargant ("witch engine") and had lost their Shadowsword variant in the first encounter. Scene occurs at a banquet aboard the command leviathan (which make Baneblades look like toys for size).
A sentence further on: "These eyes constantly scanned the room, as if searching for threats, and from time to time he would fix one or another of he men around the table with a piercing, assessing stare. All were targets of his judgement, high and lower ranks alike, and none could hold his (Brother Arnegis) gaze. The psykers at the table, especially, seemed to draw his attention."
Oh, maybe just veneration, right? IG Primaris Psykers, right? Sanctioned by the Emperor, right? Let's move on a couple of pages.
"The commissar face's <sic> stiffened at the at the approach of the powerful witch. For all that they were sanctioned, and bore the same gifts as the Emperor himself, albeit to far meaner proportions, psykers did not share His will or His purity. The Space Marine, too, watched with undisguised suspicion.
The psyker paid neither of them any heed."
WHAT?! A Black Templar suspicious of a sanctioned psyker who had been "touched by the Emperor", oh, surely it is reasonable to say Templars actually venerate such witches, isn't it? Of course, just a few minutes later, the bloody witch manages to get possessed by the head Ork weirdboy and explodes in streamers of green fire.
The next appearance of the Black Templars (since, after all, this is a coming of age/redemption story for Bannick as well as one about Baneblades, and too much Marine would get in the way) are at the end of the epic struggle, when the Mars Triumphant (the Baneblade of the title) has successfully (with some help) disabled the mostly rebuilt Shadowsword from the beginning of the book, and the witch Gargant while being destroyed itself. The fall of the Gargant drops the psychic shield protecting the main Ork infestation and allows the Black Templars to drop pod assault ("...laying intricate killing fields of fire with their boltguns.") in with orbital fire support (HEY! They read our codex at the Black Library, too bad GW is totally clueless.) to lead the purge of the vast majority of the Ork leadership and troops.
A long post, but the concept that some stray writer writing a stray book about the Imperial Guard, with some Space Marine window dressing, when choosing the Black Templar for the "dressing", immediately keys into the Abhor The Witch, Burn The WItch mindset perfectly. Why couldn't I even agree with the 'reasonable' idea about sanctioned psykers? Because this, and I think a couple of other places, again with traces of Black Templar fluff, always goes 100% against the idea that witches, of any sort, are good. Some may require acceptance by order of the Emperor, but they don't have to be trusted and certainly not be 'venerated'.