Now back to my serious mode
The reasoning in the reference 1 is pure non sense.
Because of coning the blades will see an extra variation in angles, and there is nothing wrong with that, other than the fact that a coned rotor blows back more , and typically will phase shift a bit more, to be taken into account in the delta3's in the rotor rigging to maintain controllability.
The vibration at the low speeds comes from the fact that the rotor ingests some vortices from the air it is blowing in front of the machine and that the heli is progressively overtaking. Also only a IGE effect, try it OGE...
d3