The "$2.1B" was a made up number based on some ridiculous estimated post-merger valuation. Frontier offered an all equity purchase, meaning they would give 19% of current Frontier stock plus Spirit's creditors would get $400M in "second lien debt". Frontier is only worth $900M right now, so 19% of that company is less than $200M (not $2.1B), which is why Spirit creditors turned it down.

There was no actual cash being given to anyone. It was all either stock or a note backed by issuing more stock.

So effectively they said to tell the creditors to wipe out their $2B of debt owed to them and Frontier will give them 20 cents on the dollar an even lower priority, unsecured note that was also solely backed by issuing new Frontier stock to creditors. The $2.1B is a made up number based on the stock value that clearly doesn't exist since Frontier isn't even a $1B enterprise.