This isn't exactly the first time. There have been a very few battles before where supercapital ships engaged other supercapital ships in strategic (above 10 on a side) numbers: Uemon, Asakai, Y-2 I think to a certain extent and I might be missing a few. But certainly less than 20 in the entire history of Eve.
What this was was the largest such battle with the greatest value of losses ever. This was because it was the first largest battle where the node did not crash due to all the players on it before the slaughter was done. And if I can give you a little insider info, this was because the CFC/RUS side (the people who eventually ended up victorious) went to great lengths to control the number of people in system, keeping it down to a manageable quantity for the node to survive.
They camped the other side's staging systems with most of their smaller ships, keeping both their own and the opposing pilots OUT of the system where the fight was going on. Only a token number of subcapital ships where kept around to keep their own supercaps clear of tacklers and bubblers, and they were mostly leftovers from the initial rush to secure the system and had therefore loaded the system ages ago. Subcapitals were also ordered to abandon the drones they had out in system to further reduce strain. And still, the system was nearly brought to a standstill by the fighting.