For what it's worth, Warner Brothers has both 35mm prints and a 4K DCP of Superman II - The Richard Donner Cut available for theatrical play, so the idea the upcoming disc is some sort of upscale doesn't sound logical.
Has it been confirmed that the Amazon exclusive contains individual steelbooks of each film ? The photo looks like it may be a single steelbook with the 5 films all in one case like Universal's original 4K steelbook of the first 5 Jurassic Park films.