Is it really necessary to have a lander to perform radio-astronomic observations in moon's shadow? Isn't it easier to have an orbiting spacecraft instead and perform observation while it's orbiting behind the moon?
1. James Webb is in the Earth-Sun L2 point, where it is largely (though not completely) shaded from the Sun. A radio telescope at Earth-Sun L2 wouldn't be shaded from Earth RF. [edit: JWST is in a halo orbit which keeps it out of the shadow]
2. The Earth-Moon L2 point is shaded from the Earth, but not the Sun. So no benefit compared to the far-side lunar surface.
3. According to TFA, being on the lunar surface gets the telescope out of the solar wind, which is noisy at the low radio frequencies being observed.
Another reason could be testing for building a much large radio antenna on the moon's surface in the future which is mentioned to farther down in the article. The moon itself and it's dust has electromagnetic effects that might effect measurements and learning about them now could help future planning.