Yes but the lore for the Brewmaster is forever tied as an aspect of the Monk class now.
Just like when they decided to give Frost and Blood to DK's, that forever changes what a DK is identified as. Those abilities would likely be applied in retrospect, with Arthas and co having powers over Frost and Blood during War3, just not represented directly in the game as it was back then. The lore doesn't really make a distinction to say Brewmasters aren't Monks at a certain point in time, or that DK's couldn't use Frost at X point in time. It all just gets retroactively applied, unless there is specific lore saying otherwise.
We even have lore now that does this like Shadow Hunters, Farseers, Spirit Walkers and the like all being lumped together as Shaman. Even though they are not specifically a Shaman back then, they are considered different types of Shaman now in retrospect. Another example is Priests using Shadow; even in WC3 the neutral Troll 'Shadow Priests' only used Holy magic; now lore explains all having access to real shadow magic.