The explanation of this asteroid and features presents a very large extrapolation for its past based upon limited observations and its current orbit and position, likely does not extend back 4.5 billion years ago but only more recently in the solar system.
In a period of 4.5E+9 years, Bennu could complete 3.75E+9 perihelion passages of the Sun using a period of 1.2 years. The explanation of this asteroid and features presents a very large extrapolation for its past based upon limited observations and its current orbit and position, likely does not extend back 4.5 billion years ago but only more recently in the solar system.
Half-kilometer Bennu is likely a chip off that planetesimal block, formed from the debris in a catastrophic collision that happened some 0.8 to 1.5 billion years ago. It probably belongs to one of either the Eulalia or Polona asteroid families, both of which originated in collisions at about that time, says Daniella DellaGiustina (University of Arizona). Then, sometime between a few million and a few tens of millions of years ago, gravitational interactions with the giant planets and long-term interactions of solar radiation with spinning bodies shifted Bennu to its current near-Earth orbit.
That migration through the asteroid belt allowed Bennu to collect additional rubble from other sources, such as the six Vesta-like boulders reported in Nature Astronomy.
Bennu was never a part of the main belt asteroids. ...
Bennu is a very young asteroid if the hydrated materials on its surface are still experiencing steam (or any kind of volatile) explosions. Mars has an abundance of hydrated rocks and has a relatively shallow gravity well. This would suggest that Mars or maybe Phobos was the origin for these hydrated materials.
The orbit shown below does suggest that Mars was the origin for the rubble that would become Bennu.
In view of how crater-counting is unworkable for age-dating Stickney Crater we conclude an alternate age for Stickney Crater of 0.1–0.5 Ga that is constrained instead by the boulder evidence of Thomas et al. (2000), the boulder destruction rate analysis of Basilevsky et al. (2013, 2015), and the observed space weathering of Phobos regolith (Cipriani et al., 2011; Pieters et al., 2014).
In post #4 we read "But the datings are precise..." That is one interpretation. I did not see radiometric ages published for Bennu,