Here are my files:

spawn.json

[
    {
        "incontrol": true,
        "result": "allow"
    },
    {
        "result": "deny"
    }
]

spawner.json

[
    {
        "mob": "minecraft:zombie",
        "persecond": 1,
        "attempts": 5,
        "amount": {
            "minimum": 10,
            "maximum": 40,
            "groupdistance": 10
        },
        "conditions": {
            "dimension": "minecraft:overworld",
            "norestrictions": true
        }
    }
]

If I'm not wrong, the spawner.json data makes a ton of zombies spawn without restriction (AKA during the day, any light level) and the spawn.json data makes only the InControl custom zombies able to spawn and disables any other mobs. So what I should be seeing is just a bunch of zombies wandering around with no other mobs at all.

But running this code on a new superflat world makes no mobs spawn at all, even during nighttime.

What is wrong with my code? Is there a required field that I am missing?

You got rid of a lot of fields for the 'conditions' block (like minheight and maxheight and so on). I recommend not depending on the defaults as they may not be suitable for flat worlds. Also try with only [] in spawn.json. To test if it works with spawner.json alone

Adding in minheight/maxheight/mindist/maxdist worked. Thank you so much!