There's a difference between what people think, what they believe, what they present... and additionally, what's true. Your telepath may be able to pick up on one or maybeso two of them, but the last will trip them up hard. Honesty, loyalty, competence, and effectiveness are not synonyms. Your warlord is dealing with people, and people - are just not simple. The previous differences will make a huge difference in deciding who to trust, who to listen to, and who to believe (and yeah, those aren't the same thing either).
His enemies' plans will also not be 100% set in stone. They should be calculating a lot of possibilities based on what they know, assume, see the warlord doing, and have lots of contingency plans. The telepath might see some of those plans - but everyone involved will be plotting lots of plans, and since they aren't "decided", who can say which will end up happening? There will be more than one person he is acting against in each situation (kings, generals, army leaders, advisors), they may or may not be in agreement (and if they disagree, he has to decide which plan to counter). If someone changes the plan in the middle of things, that will not be available in advance. How the enemies will react once the warlords' plans go into effect is also up in the air, they may react in unexpected ways and without time for the warlord to react to those changes. If someone on the other side doesn't inform people of the whole plan, but just parts - the people in range on the battlefield can't betray it to the telepath.
Anyway, set that aside and let's say your telepath knows when someone is lying to them. Handwave away, for the moment, all the limitations on how many can be picked up at a time, if one person can be picked out of a crowd, how deep the deception goes before it can't be picked up (because the person is lying to themselves...). He asks - can this be done? and gets answers of yes, no, if they do it this way, if you do it that way... and even if they all think they're telling the truth, who can say who is right? Or, if one was thinking about spending the coins he would get rather than filling his head with the reasons for his recommendation - did that make his answer wrong? What if one was planning to betray him - but maybe was giving good advice in the meantime, so to have a better situation to "inherit" from him? Will these things make your telepath not believe them, and what happens then if one was right, and your telepath listened to an honest fool instead? Your warlord may lose everything here, and this is not even about knowing his enemies, it is about his own soldiers, advisors, generals, and allies!
And even if the telepathy has given some game-changing secret, the warlord dare not forget that he needs the loyalty of his followers, and the agreement of his allies - expecting none of them to object (or not ally with him) because of telepathy being used on them is not likely, and just as problematic is them seeing him acting on information that they don't know where its coming from and it doesn't make sense to them, listening to people (or not) on what seems a whim, and knowing he doesn't trust them with the reasons why. Will they believe the game changing secret? Will they believe the telepath is loyal to the warlord? Do they know that's where the information comes from? Do they believe what he does based on the telepathic information is done because of facts, not whim?
Telepathy seems easy, like a shortcut. Telepathy seems useful. That's why it is a trap for your warlord. Because warlords and generals have been fighting without the advantage, and there are plausible ways, which let the greatest make themselves successful, to root out disloyalty or treason, judge which of several options is wisest, to look at ideas as more than the person who offered them.
And it would be so easy, to lean on telepathy instead, and forget (or let dull) all those necessary skills. It would be easy to pick someone enthusiastic over someone competent, to favor someone obedient over someone effective. It would be easy to favor ideas based on who is loyal and honest, or overlook ideas from those of more... ambiguous loyalties, instead of judging the ideas themselves for what will work best in a situation. Easy to believe that knowing a secret is true (via telepathy) doesn't mean having to justify it, or risk losing the faith of those who follow him.
And, yeah, in the end I think the telepathy will make it far to easy for your warlord to fail.