WD 1856 has smaller mass than WD 1856b. So how WD1856 making WD 1856b revolve in the orbit? Its clear violating space-time curvature.
WD 1856 actually has a larger mass than WD 1856 b: the NASA overview page gives 0.518 solar masses (543 Jupiter masses) for the white dwarf, while the planet has less than 13.8 Jupiter masses. The white dwarf has a smaller radius than the planet as it is much denser.
WD 1856b is not more massive than the star it orbits. The radius of WD 1856b is much larger than its star because its star is a white dwarf; but WD 1856b is much less massive. That gives the star a diameter of a bit larger than Earth while the planet's size is about that of Jupiter.
The star, WD 1856+534 is about 1/2 the mass of our Sun or about 500 Jupiter masses. The planet is listed as "less than 13.8" Jupiter masses, so the star is at minimum over 30 times the planet's mass.