Yes, that is the heart of the issue. Being able to detect something and being able to engage it are very different things, and require very different radar wave-lengths.
If I go duck hunting with my eyes closed, I will still be able to detect the ducks with my ears, but my chances of shooting one are about zero.
Large wavelength radar systems require large antennas and these are HARM magnets any time that they turn on.
There's large antennas and then there are large antennas.
Consider a VHF phased array radar whose elements are dispersed over an area, such that any particular HARM missile would only disable one element, and thus only slightly degrade the radar.
Even if VHF radar doesn't have the resolution to locate the target precisely enough for missile targeting, it CAN be used to vector fighters to the general location, which can then visually locate the aircraft, use their onboard radar (stealthyness decreases the closer the radar gets to you), or use other technology, like LIDAR or IR tracking, against which radar stealth may be useless.