He exaggerates, but it was a serious matter and involved the largest mobilization of Roman military assets since the Civil Wars. The Bato boys were capable and charismatic leaders, and the Balkans, as the Romans discovered, were a territory of numerous and hardy soldiers. First, a quick overview of the bellum Batonianum for those unfamiliar.
In 6 CE, the imperator and future emperor Tiberius assembled a force to attack the Marcomanni and invade Bohemia. He recruited among the Illyrians, who once assembled realized their own strength and their own common distaste for Roman heavy-handedness. A charismatic figure from the Bosnian region named Bato stirred them up into a frenzy and they soon took up arms in "rebellion," taking Tiberius and the Romans by surprise. The belligerents included the Bosnian Bato, of the Daesitiates, a certain Pinnes of the Breuci in Pannonia, and with him a second, non-Bosnian fellow also named Bato.
Their combined force was considerable. Our sources estimate 200,000 strong, which is surely too many, but something between 50-100k seems reasonable, and certainly the Roman response would suggest it was that sizeable. They immediately pounced on and destroyed undermanned Roman legionary posts throughout the area, killed all the Roman merchants they could round up, and set off toward more strategic targets. The Breuci headed towards the Roman fortress at Sirmium (top right of red area on the map, not to be confused with the Sirmium town in Italia) and would have taken it if not for the timely arrival of the Roman official Caecina Severus with his army from Moesia (eastern yellow area of map). He defended the fortress but with heavy losses in a knock-down drag-out struggle.
Tiberius, meanwhile, threw away any hope of continuing his campaign in Bohemia and ordered the fortress at Siscia to be protected (upper left of red area on the map). This was essentially the gateway to northeastern Italia. The rebels (I hate that term but there is nothing better) were now a bit slow to capitalize, and because Caecina Severus had returned to Moesia to protect his province, Sirmium remained under imminent threat.
Back in Rome, Augustus was very alarmed. He ordered new levies the likes of which had not been seen in several generations, since the very beginnings of his civil war(s); he recalled veteran retirees back into service from throughout Italia; he authorized the enrollment of freedmen into the levies to bolster their numbers; and he imposed new taxes to help pay for it all. He sent his literate friend Velleius Paterculus with recruits (who would later be an eye-witness of these events in his histories), he sent Germanicus up there, and he moved his personal "home base" to Ariminium, in northeast Italia, to be closer to the theater of war.
All these measures began to coalesce the next year, in 7 CE. With an infusion from Italia, Tiberius now had five legions at or near Siscia; he was joined by Caecina Severus and two more legions under Silvanus from the east, but not before that force was nearly destroyed by a vicious ambush courtesy of the Bato boys at a place called the Volcae Marshes. Velleius Paterculus describes it in detail and gives the edge, narrow as it was, to Roman discipline vs rebel inexperience. By the end of the season of 7 CE, a massive force had assembled at Siscia: ten Roman legions of heavy infantry; another 5+ legions of auxiliary troops; a gaggle of Thracian cavalry; and the equivalent of another two full legions of reactivated veterans. This was, by far, the biggest mustering of Roman military might since the struggle between Octavian and M. Antonius 40+ years before.
The rest is less interesting. Tiberius almost immediately dispersed this great gathering, having made the point I suppose, and the actual fighting from this point forward was not terribly interesting. But the response to this incident is very indicative of the seriousness of these events to the Romans and in particular to Augustus. Factors: its suddenness and ferocity; its initial successes, especially the loss of Roman citizen lives in areas which were supposed to be under Roman control and protection; a series of very respectable showings by the rebels on the battlefield early on, especially at Sirmium and at the Volcae Marshes; and the very close proximity of the whole mess to northern Italia, especially the populous and prosperous Po valley. One could probably successfully argue that the threat of the Cimbri 105-102 BCE was much greater to Italia, and their absolute route of Caepio's army at Arausio was certainly a more devastating defeat than any suffered in the bellum Batonianum. In any case, I think the intrinsic threat to Italia itself is required for Suetonius' comparison (so, therefore, the Parthian war and defeat at Carrhae would be excluded, I would argue).
Ancient sources for this conflict: Valleius Paterculus Bk II; Dio 55.29ff; Suetonius Tiberius