He doesn't say that at all. He says "there is no credible case that a person born on American soil with one American parent was clearly not a “natural born citizen", which is not the same as making a positive statement that such persons are natural born citizens. Neither does he cite any case indicating that they are.
Yes, that's what he says in the footnote. But, in the paper's second paragraph of the first page under the section titled: I. INTRODUCTION: THE ENIGMA OF THE NATURAL BORN CITIZEN CLAUSE, Solum says this...
"What is the legal significance of what we can call “the natural born citizen clause”? There is general agreement on the core of settled meaning.2 As a matter of inclusion, it is beyond dispute that anyone born on American soil with an American parent is a “natural born citizen.”3 As a matter of exclusion, anyone whose citizenship is acquired after birth as a result of “naturalization” is not a “natural born citizen.”emphasis added
I left the footnote anotations in to prove that this paragraph is found in the body of the paper, not the footnotes. In any event, compare that with my statement...
"He plainly says now, that it only takes a single citizen-parent and jus soli to effect natural-born citizenship"
The language is clear to anyone who isn't a birther, looking to parse every comma, apostrophe and colon to find support for their belief (I use the word "belief", because that's what is is, something more closely resembling a religion, than intellectual exploration), or to impeach the statements of others. It's ridiculous.