> If you overwrite a pointer with an integer value, then the integer is statistically unlikely to correspond to a valid memory address

On 64-bit systems, and even then, it depends on the system’s memory layout (I think most integer values in programs are < 2³²)

Right. It’s unlikely both because the 64-bit value space is huge and because on most systems pointers have some of the high bytes set whereas typical integer values don’t. IIRC this combination of factors is what makes conservative GCs like BoehmGC quite effective on 64-bit architectures.