"The econony needs x workers of profession y". What does it mean and how is it derived?
Full-factor employment GDP (referring not only to labor but also to capital and any other factor), results from existing factor stock interacting with production technology. It is a (quasi) "engineering" relationship, not an economic one. 

It says "if all labor and all capital were employed, given technology, output will reach its feasible maximum". It does not say that it is an optimal level in any sense. It does not allow for labor-leisure choice or lock-out (idle capital). It does not take into account whether current markets actually clear (or not). 

Still we informally accept that "more is better" so we do examine what would it take to reach this feasible maximum, or how economic interactions, incentives, institutions etc may or may not lead us to such a full-factor employment GDP.

In the case examined by the question here, it appears that an expert body has identified an "engineering" mismatch in the composition of the labor force. Namely, this body of researchers has estimated the required compostion of the labor force that can lead the economy to "feasible maximum GDP"-and then found that the actual composition does not align with the former.

As to how one can indeed estimate such "required compositions" of the labor force, well, this is too technical to go into here, but it can be done.