Very interesting article! 
It says that in order to use the longest range (and "best") modes of the HARM, an additional targeting pod is needed. For the F/A-18 that's a "TAS" pod (Targeting Avionics System). 
There's mention of it on the F/A-18C in 1996 so hopefully we will get it later in DCS, though I've heard no mention of it so far...