Either...

1) "No base class involved" is a mistranslation (or a deliberate but slight misinforming of the public for whatever reason) because Dark Knight will be a class, not a job.

2) They are somehow going to find a way to equip 'nothing' in the main hand, so people can equip souls before they equip their greatswords.

3) They are going to somehow find a way to equip both a soul and a greatsword simultaneously.

4) Souls will no longer be what define a job, blurring the lines between 'class' and 'job'.

5) They will be doing away with classes altogether.

Kiiiiiinda hoping it's #5.