Actually, you quoted the rule about "beating the Stealth check by 10," which is specifically part of the Total Concealment and Superior Cover part of the Stealth skill description. A similar rule shows up in the "Targeting What You Can't See" sidebar on page 281, but that specifically calls out the circumstances it applies to: "if you’re fighting a creature you can’t see—when a creature
is invisible, you’re blinded, or you’re fighting in darkness you
can’t see through." Just haveing concealment and making a Stealth roll does not make the creature invisible, so "Targeting What You Can't See" doesn't come into play. An enemy only has to beat the character's Stealth check with either active or passive Perception, something that shouldn't be too hard unless the character has specifically trained to take advantage of Shadow Walk.
If you're having trouble with Shadow Walking warlocks, have enemies grab in the first round. I haven't had a chance to look through KotS, so I don't know what kind of enemies or abilities are in play, but I think there are plenty of ways around a basic warlock using Shadow Walk for anything more than an effective +2 to defenses.