Hi Dhruvin,
I do not have access to that article so I am unable to understand their modelling methods.
Your question is not clear regarding distortion. Are you referring to the non uniform displacement field? This is due to the very small displacements of this model and the parabolic elements being used. If you swith to linear elements and mesh much finer this displacement field will appear much smoother.
The reason the parabolic elements are doing this is the corner nodes of a parabolic element actually have negative stiffness. This along with uniform reduced integration, and a non uniform mesh, results in this random-esque appearance of the displacement field.
Also to remember is that the displacement field is being applied to the undeformed geometry in the Mechanical application. This is not what is actually happening, the harmonic response is using the deformed shape from the static structural analysis. You can confirm this by viewing the result file in MAPDL.