When it comes to cults any sociologist will tell you willingly and knowingly get real blurry. Deprogramming ex cult members is exceptionally hard.
Tide could deprogram a conventional cultist/zealot just fine, I'm sure, even if it probably takes a while. It's the metaphysics of chaos corruption that is more likely to foil Tide's efforts. When a person is warped body, mind, and soul to best exemplify and exhort a Chaos God's principles, the person is almost definitely beyond salvation. They are only
definitely beyond salvation when they wholeheartedly accepted their god and its blessings without even a shred of regret or reluctance in any corners of their blackened hearts.
For instance, a Thousand Sons Sorcerer could, in theory, be redeemed. It sorta happened before during Rylanor's middle finger to Fulgrim. Not saying that every TSS is salvageable, but at least a few of them probably are. They've resigned themselves to their current fates, but Tide could present a worthwhile alternative path for them.
Redeeming a marine of the (current) Word Bearers? Fat chance. I doubt even a Keymind would bother after the first attempt. It would be too much effort, for one. Plus, the Eldritch Knowledge they might possess (and would probably speak of) could be potentially too dangerous even for Tide. Maybe. (Admittedly, a sufficiently advanced Floodmind could probably withstand the Eldritchness of Chaos without risking SAN damage or deviance.)
Obviously, Chaos Spawn and Daemon Princes and (most) Possessed aren't even worth the thought of trying. Chaos already has them and isn't going to let them go without Tide decking a god in the face with one tentacle while doing some sleight of hand with another.