The data sheet doesn't specify the max output including DC offsets, so it isn't obvious to me. It does specify 20Vpp into an open circuit, so if it is well designed and built then it should be able to deliver 10Vpp (or 5V peak) into a 50 Ohm load. That is only 100 mA (10V peak into 100 Ohms), which doesn't sound crazy or anything. You are measuring 12V peak into a high impedance, so I would hope it can deliver 6V peak into 50 Ohms, but that is higher than the spec so there are no guarantees.
I find it interesting that it has both 50 Ohm and 600 Ohm outputs. For most lower frequency purposes either will be fine, of course. Some of the old HP 1 MHz analog function generators have 600 Ohm outputs as well. My understanding is that 600 Ohms used to be a standard in audio gear, but I could be wrong about that.
jason