I noticed it in all modes, not just marathon. I initially associated it with marathon since thats the first thing I tried after the patch, but then after switching back to other speeds I still noticed it. In my epic game I had a new barb invading my borders every turn, at one point I had about 5 barbarians attack my holy city at once, from 4 different directions (one stack of 2), with 2 more attacking the city on the opposite side of my empire. These are mostly archers and axemen, since fog of war was lifted by the time higher tech was achieved. And this was normal, not raging mode, on Noble difficulty.
The barbarians also build a lot more cities post patch. Before I only rarely saw barbarian cities on my starting continent, they were mostly limited to unpopulated continents and undesireable tundra, but post patch, the unsettled areas all around my civilization quickly become infested with barbarian cities. Thats not such a terrible thing since if you build a small invasion force of swordsmen and axemen, it can save you from building a lot of settlers, and you'll still get more use out of the military in the long run and you will still grow while producing them.
The main gameplay difference for me has been that in the early game I need 2 or 3 defenders per city instead of just 1 (one spare while the other is healing, and one to prevent unhappiness from not being defended, since I have one or two outside the city responding to attackers as often as not), and this makes expansion much much slower since insufficiently defended cities will just be destroyed. It has less impact on the AIs performance since the AI likes to build multiple defenders anyway. The patch seems to have been aimed at slowing early expansion in a number of ways, and a slower start generally makes the game more difficult all around. Paradoxically, I find I need fewer defenders around the time that civs borders start clashing than I do before then, because the barbarians are so much more aggressive than the other civilizations.
I'm pretty sure the patch has increased the default barbarian difficult across the board, its just a coincidence that lots of people are also trying out marathon for their first game after the patch. I don't really mind, but it would be nice if barbarian difficulty was a seperate drop down configuration with shades of difficulty between none, default, and raging.