As the primary motor cortex is responsible for the initial generation of impulses that will allow for movement -- in this study, they are basing their therapy off of studies done in healthy volunteers where stimulation of healthy tDCS in healthy individuals enhanced motor learning. 

In the abstract, they specifically mention that they'll be only looking at patients whose stroke did not occur in the primary motor cortex -- I assume because, in order to recapitulate the therapy that worked for healthy volunteers, they chose to stimulate the relatively "healthy" ipsilateral motor cortex of these stroke patients, who have damage in other areas.

Patients with primary motor cortex stroke would, therefore, be ineligible for the kind of therapy -- or, at least, need to be studied and evaluated as a completely separate group. 

This is only an educated guess -- if you look at the author list and contact them, I find that investigators are more than happy to answer questions. 

Meaghan has more or less covered what I was about to reply. The primary motor cortex is probably the site of the tDCS stimulation. Inclusion of people with damage to this area would therefore likely mask or dilute the observation of any hypothesised effect by way of improved function. 

As, Meaghan says, though, it is best to get hold the paper and check that this is the case, and that the intervention did indeed involve stimulating this part of the brain.

I agree with the other responses.

When I have such a question, I usually email the article's corresponding author - they usually respond quite quickly.