Whitehorn offers no basis for speculating on what "is routine in these circumstances." As I recall, he produced no research on Scott County police procedures circa 1921. He suggests that there must have been a lineup. But criminal procedure was not as constitionalized and defendant-friendly as it is today.
We don't know (yet) how the authorities came to feel there was probable cause to arrest Bix. Presumably Sarah could only say that "a man" had done something to her. It was Duncan and Bailey who identified Bix as the man who took her into the garage. Did the police ask the two boys to view Bix and identify him, by himself or in a lineup? Or was it sufficient that they had simply named Bix on the basis of knowing him from high school? Was Sarah Ivens asked to identify him, who might well have seen the man "at close range"?
Who knows? But when Whitehorn tells us that "it is difficult to believe" this or that about what the authorities did in 1921, I'm unimpressed. It doesn't matter what he finds difficult to believe.