I believe the author is wrong on that point.
Custer had faced Indians in many previous battles, where he was almost always outnumbered. The numbers were not what he misjudged. What he did not account for was that the Indians had changed their battle tactics, shifting from a hit and run mode to a full assault mode.
Custer faced Indians in many previous battles, where he was almost always outnumbered?
Because the only battle, I recall Custer fighting during The Indian Wars was The Battle of Washita River, where Custer lead a force of 574 soldiers against an Indian encampment of 250 with maybe 100 warriors.