Cannot assign a reference that has a deleted copy constructor?

Question

I'm in a situation where I need to implement Move Constructor and Move-Assignment Operator for a class which holds a reference to an object with a deleted Copy Ctor and Copy-Assignment Operator, basically looks like this:

class MoveOnlyThing
{
public:
    MoveOnlyThing() = default;
    MoveOnlyThing(const MoveOnlyThing&) = delete;
    MoveOnlyThing& operator=(const MoveOnlyThing&) = delete;
};

class Holder
{
public:
    Holder(MoveOnlyThing& t)
    : mThing(t)
    {
    }
    
    Holder(Holder&& other)
    : mThing(other.mThing)
    {
    }
    
    Holder& operator=(Holder&& other)
    {
        mThing = other.mThing;
        return *this;
    }
    
    MoveOnlyThing& mThing;
};

Now, the problem is, the assignment of mThing = other.mThing; is emitting an error:

main.cpp:40:16: error: overload resolution selected deleted operator '='
        mThing = other.mThing;
        ~~~~~~ ^ ~~~~~~~~~~~~
main.cpp:20:12: note: candidate function has been explicitly deleted
    MoveOnlyThing& operator=(const MoveOnlyThing&) = delete;
           ^

Two questions are raised;

1. How do we deal with this? Implement the Move Constructor, and then use this to implement the Move-Assignment Operator?
2. I didn't realize that the compiler would generate a copy in this case when re-assigning an existing reference. Who can explain that?