Why does a mix of condensed milk, cocoa and coconut solidify in the fridge?
Condensed milk may not get hard in the fridge by itself, but it does firm up considerably. It's very pourable at room temperature but it's almost too viscous to pour once chilled. The crushed biscuit absorbs the most liquid components of the condensed milk, leaving those components that do firm up to do so. The cocoa would also do some of that, effectively "binding" those components of the condensed milk that would otherwise remain liquid. So what is left "unbound" solidifies.