It's not a super long video but I think you're reading in things that aren't there. What he said (paraphrasing a bit, consolidating and bold added) was
Early on Rune Master class was introduced and received negative feedback so they decided to limit things with subclasses. They've already hit the major roles and archetypes, they feel like everything has already been covered. The only reason to add a new class is if it's something if the campaign demands it. Eberron needed to have an artificer because it's part of the lore and world.
He gave the idea of the mystic and that it would only be added if a campaign demanded it. Relating that back to warlords, what campaign would demand a warlord? I can't think of any. But the very start of this, the example of the Rune Master not being added supports my position. If there were enough demand for these specialty classes that do things other classes already do, they would have considered doing them. The feedback from the UA was that people don't want that.