The F-19 denomination was never used, they skipped from F-18 to F-20. The game was based on what they believed that the stealth fighter (F-117) was going to be before it was publicly acknowledged. So maybe it wasn't trademarked. (Or maybe they trademarked everything just in case...)

Vaguely similar to the F-29 Retaliator game, that was based in the X-29 experimental fighter, but not in an existing F-29 of any kind.

Same as for any game that uses real car brands or gun manufacturers, quite a few of these companies are willing to license for certain types of game