There is an AcI: there's your accusative illam, there's your infinitive intellegi, and there's your verbum cogitandi volens. “Wanting that to be taken to mean the upper.” (The basic construction, which is a bit unusual and may be giving you trouble, is intellegere with a double accusative = “to take something to mean/to be,” like Hominem divitem intellegimus cui magna pecunia est = “We take a man to be rich who has a lot of money” etc.)
Note that the text is quoting Hugo of St. Victor, the author of a chronicle. Let's add quotation marks:
Hugo […] ita meminit gentis nostrae in Chronica sua: «Northmanni», inquit, «de Scythia inferiori» (illam procul dubio volens intelligi superiorem, quam nos Suethiam apellamus) «egressi, classe advecti in Gallias […] omnia depopulati sunt ferro et flamma.»
So, the author of the text temporarily interrupts the quote to clarify the quoted text. Hugo wrote of Scythia inferior, and your author explains how he interprets this expression, which implies the existence of a Scythia superior: He thinks: Illam [Scythiam] procul dubio [≈ sine dubio] superiorem intellegi vult, quam nos Suethiam apellamus. “He doubtless wants that Scythia to be taken to be the upper one, which we call Suethia.”
Freer:
In his chronicle, Hugo mentions our nation thus: “The Northmanni,” he says, “having left Lower Scythia [he surely considers that region to be Upper Scythia which we call Suethia] and having landed with their fleet in Gaul, devastated everything with sword and fire.”
