What I think more is a possibility of the low input limits is just protecting the silicon (well if they use it on this 13GHz model). I think it's incredibly hard to protect GHz signals. An protection diode may be ruining the signal performance. Especially at 13GHz , just an extra trace to a diode can ruin the input circuitry.
A class mate of mine got a HP network analyzer for free from college. The RF output was broken and the power supply failed.He fixed the power supply, some feedback opto's were dead. RF output was broken because some amplifier shorted out one of it rails, and the bond wires were gone. Someone probably connected an output signal of a network to the RF output. Bad luck, the RF module only costs a couple of thousand dollars. Ah well, he has a nice spectrum analyzer now though.
So, RF equipment are just very sensitive. Like, you also don't turn on a RF modem without an antenna right?
+/-5V sounds sufficient for differential signals, which I think this scope is designed for (measuring quality of transmission lines)
Whilst Dave got it, maybe he can do some signal testing with his Marconi function generator, and transmission lines?
Just watch out for overshoot