In Pythagorean Library, a complete collection of the surviving fragments from the Pythagoreans, the initials fragments are those that belong to Hierocles, and in several of them, between parentheses — likely as an explanation/English-analogue to those Greek "proverbs" —, it is mentioned "God" (in capital G and singular, instead of "gods"). To my knowledge, polytheism was very much alive at that time and Hierocles was not known for being a Christian. So, are the mention of "God" there in the books an adulteration or what?
The source in question contains a grave mistake. The Pythagorean precepts (σύμβολα) pose a complicated challenge to the history of philosophy. They are traced back to Alexander Polyhistor, who notes that he himself is a transmitter of them. One may refer to (1) for a collection of them in the original and translated languages. Lest we delve into the labyrinths of doxography, I shall be content with stating just that they are consonant with the polytheism of Antiquity and are not handed down from Hierocles of Alexandria.
At present, we are aware of two works of Hierocles, and no other. One of them is his commentary on the Pythagorean Golden Verses (Χρυσά Έπη, a.k.a. Carmen aureum by their Latin title) which we have in full; the original text is given in (2). The other one is On Providence (Περί Πρόνοιας) extant in fragments and summary in Photius’ Bibliotheca (Βιβλιοθήκη).
Though Hierocles is not a Neoplatonic philosopher per excellentiam, he adheres to the core Neoplatonism. Seemingly, the characteristic henology is replaced by a demiurge.
Generally speaking, it ought to be kept in mind that the One of Neoplatonism must not be conceived in a simple numerical sense and conflated with the God of the Abrahamic religions (some translators have distorted the distinction, to be sure). As for Hierocles, the following from Hermann Schibli's eloquent entry "Hierocles of Alexandria" illuminates the point:
That Hierocles forgoes a Plotinian union of the soul with the One can thus be explained from his strict notion of the cosmic order and not because his ontological hierarchy is lacking the Neoplatonic One from which all things emanate. It is true that in his extant writings Hierocles does not explicitly mention the One. This omission has led some scholars to suppose that Hierocles represented an “Alexandrian,” pre-Plotinian brand of Neoplatonism, in which the demiurge is the absolutely highest principle and which thereby approximates Christian monotheism. The major obstacle to this theory, however, is that Hierocles (Comm. 20.12–19) unambiguously identifies the demiurge with the Pythagorean symbol of the number 4, called the tetractys or tetrad (the paradigmatic model of the physical world), which is itself a derived principle (from the monad, the principle of number). The demiurge/tetractys therefore represents a subordinate hypostasis and the One may be assumed to stand in the background of Hierocles’s metaphysics as the very first transcendent principle, in harmony with the philosophy of Ammonius and Hierocles’s own teacher, Plutarch of Athens. It is understandable that Hierocles did not introduce the One in his Commentary, since that work is chiefly an ethical treatise that was not required “to extend to the whole of philosophy” (Comm. 27.10). As for the On Providence, of which we have only meagre remnants anyhow, the subject matter would not necessarily entail a discussion of the One (just as Plotinus’s Enneads 3.2–3, “On Providence,” do not include the One).
For an in-depth study of Hierocles' Commentary, I would recommend Schibli's (3).
(1) The life of Pythagoras, with his Symbols and Golden verses. Together with the life of Hierocles, and his commentaries upon the verses. Collected out of the choicest manuscripts, and tr. into French, with annotations. André Dacier, translated by Nicholas Rowe. London, printed for J. Tonson, 1707. Freely available at https://archive.org/.
(2) Hierocles in aureum pythagoreorum carmen commentarius. Edited by Friedrich W. Köhler. Stuttgart: Teubner, 1974. Freely available at https://archive.org/.
(3) Hierocles of Alexandria. Hermann S. Schibli. New York: Oxford University Press, 2002.