Can it be possible that the same is true here for the sack of Rome? As I see it there are a few different things to consider: Something happened between a band of mostly Gallic soldiers and the Romans around that time that halted or slowed Roman ascendancy for a time, and that left an indelible mark on the Roman psyche, so that the Romans always had a sense of fear and anger towards Gauls. There are no contemporary accounts of it because there are almost no contemporary accounts of any of Roman history at this time. Later Roman annalists, writers, politicians, etc. played it up and possibly exaggerated it, and mixed in a lot of "lessons," cliches and fabrications into the common story over time. And that there is little to no archaeological evidence that it actually happened.
Finally, we have the common modern suggestion that this was actually a band of mercenaries passing through and plundering, rather than a migratory tribe. Assuming that's the case, it seems to me here that the safest conjecture is that there was a sack of some sort, focused on plundering rather than destroying. This is something that would particularly make sense if this was a band of mercenaries, who would be more interested in gold and other valuables than destruction. It would also play into the common story told between Brennus's Gauls and the weighted scales-which also suggests that those who sacked the city were primarily interested in gold. In that case, there might not be much archaeological evidence of the sack, which would fit our current dearth of such evidence.
From there you can say it's size, scale, and impact were exaggerated by subsequent generations of Romans, who also invented much of the details. This seems like a far more reasonable explanation than calling into question whether Rome was actually sacked at all. If this is the point you were trying to make, I apologize for the long winded tangent, but it wasn't entirely clear. Though I will say, when a lot of people mention "sack" on here, I assume it includes just a plundering, and they're not always referring specifically to great physical destruction (as, as you mentioned, was also the case in the Gothic sack of Rome).