That actually seems like it will be relatively small of a park...5million square feet is about 115 acres. Which is about 1/3 the size of Cedar Point or Kings Island. Hope they set aside room for expansion.
To answer your question how it is going to help is easy. It will be helping the debt significantly, in many positive ways.
Six Flags isn't paying to build this park, Dubai Holding is paying for the park and its operation. Six Flags will be helping with designing and planning as well as running the park. They are being paid to license their brand to this company, and for operating service. so basically Six Flags spends 0 dollars and shares the profits. It's the perfect move.
Also now Six Flags gets to finally do what they want and create a park from the ground up meaning they aren't spending all this time and money trying to get a broken down old dirty park in order. They will really get to show off areas of creativity and themeing design we haven't seen before. This will help contribute to strengthening the Six Flags Brand globally.
It's a very positive financial move for the company.
Funny how Busch's Dubai project was big headline news, they did the major announcement and all that, but Six Flags has kept this relatively quiet. Is it because (a) they chose not to budget for a big media release, or (b) they want to keep it on the quiet side so shareholders, employees, and others in the US that have a concern for the company's solvency don't see this and think Six Flags is blowing another wad they don't have? Even if it is only a licensing and not their money being spent on development, people see things and miss the details and form opinions based on wrong information.