MGS4 is not 1920x1080, it's not even "1080p". It is 1024x768 scaled to 1280x720.

In either case, the game not being a "regular" PS3 game makes it 1080p more likely, not less. PS2 games have lower texture resolutions, fewer polygons and fewer "special effects", making the games easier to run at a higher resolution than "real" PS3 games.

Graphical resolution is normally a technical limitation, not a budget limitation.