Russ had the exact same belief as Jaghatai. Exact same. Well, except that Jaghatai knew that his Stormseers were using the Warp and said so, while Russ insisted that they were using their own source.

The point was that 2/3 of the Librarius was arguing that the Warp was necessary to use, but needed restraint. Instead of Russ trying to stand up for that side, he just branded the whole thing as witchcraft and pretended like he and his Rune Priests were up on a pedestal. He claimed to have complete loyalty in all things, including the Edict, while allowing Rune Priests to keep doing what they were doing.

Other Legions banned their Librarius, even though they knew restraint; they acted more like Rune Priests and Stormseers in how much Warp was used and with how much care, than the Thousand Sons. When they were forced to use their psykers again, they admitted that they were going against the Edict. Guilliman said that circumstances had changed, and the Emperor would allow it now. Jaghatai just straight up said he wasn't going to listen to the Emperor from day one. Dorn was torn because of his need to obey. Russ, self-proclaimed most loyal, was never torn. He made up loopholes for his psykers.

That's his brand of hypocrisy. He later admitted that he was a hypocrite, and should have taken the stance that Jaghatai and Sanguinius had. For me, it's not really about Magnus. Russ fucked up in a different way when it came to Magnus. He fucked up with the Edict in this way.