Keep in mind that these are parking spaces at Westgate and not the Cardinals' stadium. The ones they are specifically referring to are north of Maryland Avenue and shouldn't really be theirs to claim. Glendale also gave the Cardinals' permission to develop the areas they already owned and acquired next to 91st Ave. AFAIK, the Cardinals have never used these extra parking spaces except for the Superbowl, which is part of why Glendale demolished them in favor of more development. This potential lawsuit is likely just a CYA to ensure that there is a suitable replacement for 2015.
Like the article mentions, the original plan was to build garages instead of regular flat parking but with the way the market died and Ellman halted Westgate, that never materialized.