Yes but you've all been trained in similar if not the exact same way...the leader has the necessary knowledge of military procedures, tactics, weaponry, etc. could a Project Manager whose great at leading IT people come in and lead soldiers while they were under fire successfully? Or are we saying the Warlord is just knowledgeable enough and good enough at everything from sneaking to melee combat to spells that he can lead such a disparate group of people?
An Army IT project manager? Yes.
For a long time the military would have said No, but in the last 20 years or so that has really started to change. They've become much better at teaching leadership, recognizing leadership, and utilizing leadership - regardless of MOS (or AFSC in the Air Force).
Leadership is a skill that's applicable in any situation; and while there are some differences between civilian and military leadership, they are far less than you might think.
Now I'm by no means saying that a Leader doesn't have to have any understanding of the "jobs" those they lead are doing. But Expertise in their subordinates jobs is most certainly not a necessity.
A combat leader obviously needs to understand tactics. Depending on their level of leadership (rank/responsibility), they may also need proficiency in strategy. But does a combat leader need to be able to shoot as well as their soldiers? Know how to fix a tank in the field? Know how to calculate targeting parameters for a piece of artillery?
No.
But they can certainly inspire those that do to be better at it than they otherwise would.
Same thing with a Warlord.
Does a Warlord know how to pick a lock? Is a Warlord capable of suddenly dashing across the battlefield to help an ally? Can a Warlord cast spells or have the first inkling of how magic works?
No.
But they can certainly inspire a thief to pick a lock under pressure. They can motivate an assassin to dash across the battlefield - even when they've used their actions for the round - to back up a besieged ally. And they can definitely assist in the most tactically efficient, supportive, and effective use of magic - like a commander directing artillery fire.
Now it certainly doesn't hurt if a leader is also an expert in what their people do, but it's not a necessity. Having that expertise can help initially, but usually only as a way to facilitate personal authority. In the long run, it doesn't matter.
I realize though that I just introduced something into the conversation that I haven't covered before: Personal Authority.
Don't anybody go freaking out over the word "Authority", or start worrying that this means a Warlord character can tell all of the other character what to do.
A Leader can possess two types of authority. Having both is best, though not necessary, and one by itself is stronger than the other.
First, there's Positional Authority. This is authority based on rank or position. In D&D it could be
"He's in charge because the King (DM) said he was."
Second, there's Personal Authority (also known as "Earned" Authority). This is authority based on the respect or esteem that others have for you. This one is something that has to be cultivated. It's earned through competence and inspiring trust. It's the hardest to achieve, but the strongest authority one can have.
I like to view D&D Warlord authority as Personal Authority; though it can be Positional if that's how one wants to roleplay it. A Warlord with Personal Authority is able to direct their allies because they trust them. They don't feel they are being told what to do, they've just seen first hand the benefits of doing so.
The way we've been designing the leadership features in the Warlording the Fighter thread, is that the other characters are under no requirement to do what the Warlord directs. However, if they don't, then the group does not gain the benefits that would result. In fact, if the Warlord can't inspire their group to follow their directions, it's the Warlord that's penalized through loss of their actions.
Though the argument could be made that the group also suffers if the Warlord loses their actions - but that's what happens with bad or failed leadership - lack of synergy.