The recent Phys.org piece Mass anomaly detected under the moon's largest crater is a review of a recently published and paywalled Geophysical Research Letter Deep Structure of the Lunar South Pole‐Aitken Basin .
Early on it says:
"Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That's roughly how much unexpected mass we detected," said lead author Peter B. James, Ph.D., assistant professor of planetary geophysics in Baylor's College of Arts & Sciences.The crater itself is oval-shaped, as wide as 2,000 kilometers—roughly the distance between Waco, Texas, and Washington, D.C.—and several miles deep. Despite its size, it cannot be seen from Earth because it is on the far side of the Moon.
Later in the Phys.org article:
The dense mass—"whatever it is, wherever it came from"—is weighing the basin floor downward by more than half a mile, he said. Computer simulations of large asteroid impacts suggest that, under the right conditions, an iron-nickel core of an asteroid may be dispersed into the upper mantle (the layer between the Moon's crust and core) during an impact.
"We did the math and showed that a sufficiently dispersed core of the asteroid that made the impact could remain suspended in the Moon's mantle until the present day, rather than sinking to the Moon's core," James said.
Another possibility is that the large mass might be a concentration of dense oxides associated with the last stage of lunar magma ocean solidification.
@DavidHammen's answer to a related question elaborates on some of the assumptions in the research:
Combining the gravity and topography models required two key assumptions regarding the subsurface material in the South Pole-Aitken basin. One was that varying thicknesses of crustal material lay over mantle material, with the crustal material being less dense than the mantle material. The other assumption was that the materials were in isostatic equilibrium. This latter assumption may not be valid; it is well established that the large lunar mass concentrations outside the polar regions are in a super-isostatic state. That said, the South Polar-Aitken basin does not exhibit the signs of being in a super-isostatic state.
There is a lot of geophysics going on here that I don't understand.
But the statement in the Phys.org quote about heavy material being "sufficiently dispersed" so as not to sink to the core seems to be the opposite of "the materials were in isostatic equilibrium". Is it possible to reconcile these two? Am I comparing apples to oranges?