Yes it does. But “standalone mixer” does not mean external mixing of software decks. It just means it can mix external sources (line/phono) without the laptop.
Actually, this has nothing to do with the type of sound card used in the controller but the way audio is being routed by the controller.
As kradcliffe said, it’s not something that can be changed by using another software. MC6000mk1 has a switch on the back of the unit labeled “PC Mix/INT Mix” that determines how the audio/midi path inside the controller is routed. When INT Mix is selected, the mixer section of the unit stops sending/receiving midi messages to the software (so it cannot mess with the internal software mixer and lights on the units mixer section) and audio from usb soundcard’s outputs is routed through the mixer section (instead of the outputs+headphone) so hardware eq, cue and faders can be applied.
So, on the one hand you get lower amount of midi messages sent/received (you can get lower latency in software?) but you loose the ability to select how your EQ will be (full kill or -26db and crossover points - Traktor has many options for example) and, probably the thing that most people cared about - you loose the ability to have post-fader software FX (no echo tails when you lower the linefader).
The only two controllers that can work like that are MC6000mk1 and DDJ-SZ (and the SZ cannot change that - that’s why you will see countless posts on Serato/Pioneer forums from people complaing about why their flagship controller cannot have software echo trails - and that’s why they have released SZ2 version).
And most important thing: if shouldn’t affect sound quality. Make sure you have Traktor settings right (EQ mode for example).