About magic in Midnight Tides

So it is pretty clear for the most part mages are using Hold based magic in this book. But it's not like they're not aware of warrens:

Binadas came to his side. 'This was born of a warren,' he said.

Trull nodded. Sorcery. Nothing else made sense. 'A god.'e,

Now I'm on chapter sixteen and we meet Iron Bars, a Crimson Guard and he says his company has been stuck on Letheras for the past two years cause Corlo, their mage didn't find a warren until now, because of something Edur did?

If he noticed he gave nothing away. 'My Blade's waiting. Corlo's managed to find a warren in this damned Hood-pit – first time in the two years we been stuck here. What the Edur brought, he says. That's why.'

I'm assuming it might be the fragment of Kurald the Warlock King was talking about, but I'm not sure.

But what does this even mean? I thought people had access to their warrens regardless of physical locations. How would the presence of a warren nearby (if that is what it is meant as in the text), allow a mage to use their own warren? Do warrens have no inherent presence on a continent where Hold based magic is prominent? I thought they were accessible all over the world because of what K'rul did.