Pope St. Nicholas certainly believed it was historically true. Not only did he testify to the legitimacy of the case, but he even appealed to it as an historical precedent for the doctrine that 'the first see is judged by no one'. In his letter to the Emperor Michael, he wrote:
“In the reign of the sovereigns Diocletian and Maximan, Marcellinus, Bishop of the city of Rome, who afterward became an illustrious martyr, was so persecuted by the Pagans that he entered one of their temples, and there offered incense. Because of this act, an inquiry was held by a number of bishops in Council (the Councilio of Sinuessanensi), and the Pontiff confessed his fall. None of them ventured to pronounce sentence upon him, but they all proclaimed: ‘judge your cause with your own mouth, we will not judge you’; and again: ‘Let not,’ they said, ‘our judgment be heard, but you judge your own cause’; and again, ‘for of thyself thou shalt be justified, or of thyself condemned, by thy own mouth: The first see is judged by no one’.” (Pope Nicholas I, Proposueramus Quidem, ad Michaelem Imperatorem, Anno 865, p. 940)
The Liber Pontificalis of Pope Damasus (AD 366-384) relates the following about Marcellinus:
"XXX. Marcellinus (296-304)
"Marcellinus, by nationality a Roman, son of Projectus, occupied the see 8 years, 2 months and 25 days. He was bishop in the time of Diocletian and Maximian, from July I in the 6th consulship of Diocletian and the 2nd of Constantius (a.d. 296) until the year when Diocletian was consul for the 9th time and Maximian for the 8th (a.d. 304). At that time was a great persecution, so that within 30 days 17,000 Christians of both sexes in divers provinces were crowned with martyrdom.
"For this reason Marcellinus himself was haled to sacrifice, that he might offer incense, and he did it. And after a few days, inspired by penitence, he was beheaded by the same Diocletian and crowned with martyrdom for the faith of Christ in company with Claudius and Cyrinus and Antoninus, and the blessed Marcellinus on his way to his passion adjured Marcellus, the priest, that he should not fulfil the commands of Diocletian. And afterwards the holy bodies lay in the street for an example to the Christians 26 days by order of Diocletian.
"Then the priest Marcellus and the other priests and the deacons took up the bodies by night with hymns and buried them on the Via Salaria in the cemetery of Priscilla in a chamber which is well known unto this day, as Marcellinus himself had commanded…” (Liber Pontificalis)
Bellarmine accepted the account of Marcellinus as an historical fact, and cited the authority of Pope Nicholas and Pope Damasus as evidence for it. In Book IV of De Romano Pontifice, he writes:
Dollinger: “The conduct of the Popes since Innocent I and Zosimus, in constantly quoting the Sardican canon on appeals as a canon of Nice, cannot be exactly ascribed to conscious fraud — the arrangement of their collection of canons misled them. There was more deliberate purpose in inserting in the Roman manuscript of the sixth Nicene canon, "The Roman Church always had the primacy," of which there is no syllable in the original, — a fraud exposed at the Council of Chalcedon, to the confusion of the Roman legates, by reading the original.“Towards the end of the fifth and beginning of the sixth century, the process of forgeries and fictions in the interests of Rome was actively carried on there. Then began the compilation of spurious acts of Roman martyrs, which was continued for some centuries, and which modem criticism, even at Rome, has been obliged to give up, as, for instance, is done by Papebroch, Euinart, Orsi, and Saccarelli. The fabulous story of the conversion and baptism of Constantino was invented to glorify the Church of Rome, and make Pope Silvester appear a worker of miracles. Then the inviolability of the Pope had to be established, and the principle that he cannot be judged by any human tribunal, but only by himself. … So the acts of the Council of Sinuessa and the legend of Pope Marcellinus were invented, and the "Constitution of Silvester," viz., the decision of a Synod of 284 bishops, pretended to have been held by him in 321 at Rome, evidently compiled while the bloody scenes in which clerics were murdered or executed for their crimes were fresh in men's minds. There again the principle was inculcated that ‘no one can judge the first See.’ (The Church and the Council, Janus)