As for what Daniels wrote in Superman: The Complete History, he may not have been allowed by DC (or their lawyers) to say that Siegel wrote the story, particularly if the book was commissioned by DC.
Isn't that ridiculous? He either did or he didn't. Love him or hate him, facts are facts.
But that's just the point, DC are not interested in the fact of whether or not Siegel wrote the first Superboy story, they have steadfastly denied him and Joe Shuster credit for the creation of Superboy and are not going to admit in print that they did (and certainly not in any book that has been commissioned or approved by them), as they may potentially lose many millions in their current legal wrangle with Siegel's heirs.
That is why, at the start of each episode of
Smallville appears the carefully phrased byline: "
Superman created by Jerry Siegel & Joe Shuster", not
"Superboy created by Jerry Siegel & Joe Shuster".
People who are more knowledgeable (or have access to the facts) than I am in this area seem to be generally in consensus that Siegel wrote the first Superboy story, which is good enough for me, although admittedly I'd be more comfortable with citations of their source or sources, but one very rarely sees footnotes, even in scholarly publications about comics.
It is of course quite possible that part of the story at least may have been rewritten
by someone else (maybe the part that you, Aldous, think to be un-Siegel-like), more likely than not by the editor, in this case, Jack Schiff.