IIRC, they also discuss the heat of earth re-entry from such a high speed descent is greater than any current heat shield protection would allow.
Dragon's heat shield is already designed for Mars re-entry. Using PICA or PICA-X it's not much of a problem.
The entry velocity would be between 15 and 21 km/sec, versus a lunar entry velocity of 11 km/sec or a low-orbit entry velocity of 7.9 km/sec, and the re-entry window is easy to hit even with mid-1960's technology.
1966 NASA report on re-entry from Mars