Mixed arms Yes. This naturally happens when you have infantry/cav/artilery though. I guess its referring to more emphasis on units working closer together rather than operating independently away from other units and leadership bubbles?
before I forget:
I feel that Sylvan Elves should be the elves that are least mixed arms. Like their units are pretty darn good at operating independently all over the board - characters do not buff units except fluffy type buffs , almost no bubble effects beyond magic.
HBE being in middle somewhere, they can have individual mixed arms stuff like fae miasma, phoenix debuff type stuff, characters buff the unit they are in, but very little bubble effects beyond the cover volley fire and magic.
Dread Elves should be the most mixed arms with units relying on buffs and bubbles. Academy, Alter, and Beastmaster bubble effects being prime examples.