Why are modern telephone lightning arrestors less prone to (rectification) RFI problems than old ones?
There are many different kinds of lighting arrestors -- and all of them work differently, age differently, and fail differently.

A lighting arrestor diverts the high voltage lighting strike to ground instead of allowing the voltage to pass through to your radio equipment. 

Some are simply devices that are basically spark plugs.  High voltage jumps across the electrodes and goes to ground while low voltage does not.  Others consist of gas-filled tubes that ionize at high voltage and conduct while remaining inert and non conductive at lower voltages.  Many today are solid state devices such as a series of MOVs. 

After taking a lighting hit, the electrodes can be damaged or melted.  The gas may heat up, expand, and create a leak due to the pressure.  The junctions in the solid state devices may be damaged.

You can argue that the most dangerous failure is that they no longer send high voltage to ground, but there certainly are failure modes that can cause them to have high impedance, blocking RF, and in some cases rectifying it like a diode as they conduct part of the waveform to ground. 

Many advise replacing the lighting arrestor after each lightning hit.  Some also have an estimated life after which they should be replaced. 

I don't know if I agree with a blanket statement that new lighting arrestor designs are better than older designs, but I would agree that a new manufactured arrestor is going to be better than one that has been in use for many years.