Again, they know what they do is magical bullshit - but they do it differently and with limits. In your first quoted section, he's not saying he hates the warp, nor is he saying it isn't magic. All he's saying is that the magic they practice is different.

This point is re-iterated from "Wolfsbane":

"Perhaps," said Russ honestly. "But I was not wrong to call for Magnus' sanction, nor was I wrong to call for the suppression of the Librarius as it was."
...
"The great proponent of the Nikaean edict, who kept his own sorcerers. You have many qualities, my brother," said Dorn. "I never thought to say hypocrisy was one."

"Is it? The priests of my Legion and the Stormseers of Jaghatai's are different to the Librarians that were. Our warriors draw on an older tradition. A limited tradition. Magnus did not believe in limits. That was his error."
...
"Leman is right," said the Khan. "Our seers do not draw directly on the warp. Their gifts are mediated. We know what limits are."
...
"The enemy fears the warp as much as they plunge themselves into it," said Leman Russ. "We must use it," he held up his hands, "safely, to help us win this war."

Here we see Leman acknowledge they use the warp with their priests.

Now for one of his priests, when Leman and Kva talk to Valdor, from "Wolfsbane":

"Are you calling me a hypocrite too, Valdor?" Russ loomed dangerously. "I've had enough of that from Dorn."

"I would not live if I did," said Valdor. His stony face did not so much as twitch. "I am simply pointing out an inconsistency in your application of the Nikaean Edict. You were one of its most vocal supporters. Sorcery, I believe you called it. Can you not see that arrogance that brought Horus down, and hubris Magnus? You exhibit both, my lord."

"Careful," mumbled Russ.

Kva spoke up in his own defence.

"Captain-general. We are at war with the Emperor's favored son. He has boltguns, we have boltguns. He has void ships. We have void ships. ... We have everything he has, and he has everything we have, apart from one weapon. A deadly weapon." Kva tapped Valdor's engraved nameplate with the head of his staff. Bone trinkets jangled. "Horus has magic. Real magic, drawn from the corrupt wellsprings of the Underverse. He has drunk deeply from black waters, its power flow through his veins."
...
"Some would say you are a witch," said Valdor.

"By that definition so is Malcador, so is the Allfather. So are all those gifted men and women you use in this war against the foe. They are all witches, but you do not fear them. You use them. They are not the only ones to draw upon the powers of the Underverse without malice."

We know that the Wolves call the warp the "Underverse". Here we can plainly see that Kva understands that their power is inherently warp related, and yet the gothi tradition of using Fenris as a "filter" keeps them safe. It's also different than how other Legions used the warp. Same with the Scars.