No. The HARM can use
ANY part of the radar beam, including and especially the
SIDELOBES.
The HARM missile has a terminal homing capability that provides a launch and leave capability for the launch aircraft. Additional unique features include the high speed, low smoke, rocket motor and seeker sensitivity that enable the missile to easily attack sidelobes and backlobes of an emitter.
A radar beam have multiple components and not all of the beam is used. Your illustration have sidelobes that most radar systems do not use because the sidelobes are weak and usually contaminated. But an ARM attacker does not care. The sidelobes are good enough as a source.
Fivenines of antennas leaks to the rear and those leakage signals are called 'backlobes'.
apps.dtic.mil
Four modified horn antennas with greatly reduced backlobes are described. The principal method of reducing backlobes consists of preventing the energy from illuminating the edges from which it is diffracted into the back regions.
That mean even if the antenna is pointed away from the SEAD/DEAD fighter, the antenna's backlobes are sufficiently strong enough for the HARM to use. The backlobes create the SEAD/DEAD diversionary tactic where one fighter is out of missile range but within radar range and the other fighter attack the backlobes.
The newer HARM today have their own millimeter wave radar to perform their own beamriding. All HARMs are beamriders no matter their source transmissions.
Please...We are masters at this...