Isle of the Unknown NPCs, monsters, etc. all have 'non-standard' abilities and configurations. Nothing looks like a Goblin, or cast spells like a Magic-user, etc. Nor are there renamed substitutes for the standard array of such creatures.
Why is this element of surprise or novelty so important?
Why are such elements such as details about the human inhabitants, basic stat blocks, etc so irrelevant?
While it does seem true that you want Isle of the Unknown to be used, as a working supplement for a campaign, it does seem like you are asking your audience to engage in a different style of play then that which they are used to. That seems to move Isle of the Unknown from the realm of 'useful gaming supplement' into the realm or art, where you 'ask the audience to see things your way, not their way' to paraphrase Rothko.
Is this true? Do you want people to see and play D&D in a different way?