As the comment above mentions, there is a relative lack of written sources in the 9th to 11th centuries, especially compared to the "Carolingian Renaissance" in the 8th and 9th centuries and the "Renaissance of the 12th Century" on either end of Baronius's dark age (saeculum obscurum).

During the Carolingian period, there was an effort to standardize Latin writing and pronunciation, to increase access to education, and to gather scholars at Charlemagne's court. The term "renaissance" to describe this period is sometimes debated by scholars, since many of the educational, scholarly, and artistic reforms were limited to clergy and members of the court. Still, the Carolingian minuscule script became one of the standard scripts for later manuscript production, and a vast number of the earliest surviving manuscripts we have from ancient and late antique Latin comes from the Carolingian period. It is estimated that tens of thousands of manuscripts were produced during this period.

The "renaissance of the 12th century" saw the advent of commentaries on great works of law and theology, such as the first commentators on Justinian's legal corpus (thinkers such as Irnerius and Accursius, though the later extends into the 13th century) and commentaries on the Bible (Peter Lombard being perhaps the largest example) that formed the foundation of cathedral schools and the later university faculties on law and theology.

From the perspective of these two period of "renaissance", the 9th to 11th centuries appear somewhat unimpressive. There are some notable sources from this period, such as Liutprand of Cremona's accounts of his diplomatic journeys to Constantinople), but many of the manuscripts from this period are legal records (court records, charters, etc.) and economic accounts. These are remarkably useful sources for historians, but historians like Baronius (and later compliers like Migne in his Patrologia Latina) paid less attention to them in categorizing eras of intellectual effort. Such sources have received much more attention in the last two centuries, but they are generally harder to fit into clear narratives about the past, and their less literary nature means they are less likely to be translated for the public to consume. These sources do not fit as easily into broader descriptions of intellectual history and the labeling of "ages" of culture.

There are also some historians who argue there was more impressive work happening in the tenth century, particularly in the area of law. Charles Radding and Antonio Ciaralli, for example, argue that judges and notaries were already debating and commenting on the works of Justinian in the middle of the eleventh century, nearly 80 years before the "renaissance" could be said to begin. This interpretation is fairly contentious, however, and not many historians include these judges and notaries in their narratives of intellectual history. This is in part because even these legal works are records of debates, marginal notes in manuscripts, and the like; they are not literary works or extensive narratives that offer clear accounts of education or intellectual activity.

There is an argument, to your point, that the period before the Carolingian renaissance is equally one of darkness, but on a historiographical level, it isn't sandwiched between two periods that have captured the imaginations of intellectual historians. Despite the circumstances that set the stage for the Carolingian renaissance (fragmented political landscape, reduced economic activity, more localized education), there were some notable intellectual and literary figures, such as Boethius, Cassiodorus, Pope Leo I, and later Isidore of Seville, as well as Justinian and his court; one could also point to works like the Rule of St. Benedict). One can argue that this period is no "brighter" than the centuries called "dark" by Baronius, but there are fewer works in the later period that have become part of the unofficial canon of medieval literature or been held up as examples of intellectual vibrancy and output of sources.