C.N. Nam cum Byzantio expugnato

Cornelius Nepos: Pausanias

Context: Pausanias, after taking Byzantium, plans to make contact with Xerxes

Nam cum Byzantio expugnato cepisset complures Persarum nobiles atque in his nonnullos regis propinquos, hos clam Xerxi remisit, simulans ex vinclis publicis effugisse, et cum his Gongylum Eretriensem, qui litteras regi redderet, in quibus haec fuisse scripta Thucydides memoriae prodidit. . . .

This quotation affords a good workout on sequence-of-tense, but here I want to concentrate my question on the last bit.

Gongylus brought a letter (litteras) to the king

in quibus haec fuisse scripta Thucydides memoriae prodidit. . . .

“in which, Thucydides said, these words appeared. . . .”

haec . . . scripta: accusative plural, the subject of fuisse
fuisse: perfect infinitive

This looks like plain old indirect discourse with subject-accusative and infinitive. Unless I am mistaken, it matches James Morwood’s statement of the rule, for the tense of the
infinitive fuisse: “the infinitive is in the tense of the words actually spoken or thought.

We have C.N. summarizing Thucydides, so the words actually spoken or thought, would be the words of Thyucydides, and my presumption C.N. means to represent Thucydides writing in some kind of past tense.

That is correct. The past tense of Thucydides’ words was the past tense of his lifetime as compared to the present of Nepos’ own present day.