So, comparing this to BEM:
BEM: .module__submodule
SMACSS 2.0: .module--submodule

BEM .module--modifier
SMACSS 2.0: .module-modifier

BEM .module__submodule--modifier
SMACSS 2.0: .module--submodule-modifier

The difference being submodules are denoted with a - instead of a __ and modifiers are - instead of -- ?

And the notion of a submodule is the same as a modifier?

And the notion of a submodule is the same as a modifier?

Yup, he basically mapped BEM to this:

B : moduleName
E : moduleName--subComponent
M : moduleName-subModule

It's less jarring.

Also it means I can go back to camel casing :) (I never got used to -'s in class names)
.someFoo--subFoo

I like this a lot, but I'm not quite sure what the difference is between a --subComponent and -submodule as far as structure is concerned. They both kind of seem the same to me depending the context.

In this variation, a submodule is a variation on a module. So .button would be a module with .button-primary being the submodule. I prefer the term "modifier". "Submodule" is a curious term to use as it implies a descendent relationship.

A subcomponent would be something like:

<div class="modal">
    <!-- .modal--header is a subcomponent -->
    <div class="modal--header">Modal</div>
    <div class="modal--body">
        ...
    </div>
    <div class="modal--footer">
        <!-- .btn-primary is a submodule -->
        <a href="#" class="btn btn-primary">Save</a>
        <a href="#" class="btn btn-cancel">Cancel</a>
    </div>
</div>

More Transparent UI Code with Namespaces has some interesting ideas on adding namespace prefixes too.