This is a mistake on my part, and will be fixed later today. As for the point of the constraint, it is mentioned in std.internal.math.biguintcore:

    Idioms:
      Many functions in this module use
      'func(Tulong)(Tulong x) if (is(Tulong == ulong))' rather than 'func(ulong x)'
      in order to disable implicit conversion.

I believe the correct version should be

    static BigUint divInt(T)(BigUint x, T y_) pure if ( is(Unqual!T == uint) )