Instructions on how to do the trick (shamelessly copying splax post):
1. Line up real hts next to hallucination
2. Mark all the real hts and set to a ctrl group
3. Mark all the hts (hallu + real)
4. Quickly set the same control group, select the group and press merge (i.e "ctrl + 1, 1, r")
The real HTs need to be newer than the halluc HTs in order for the archon to be real. i.e. Make halluc HTs, then merge them with real HTs that just came out of gateways.
FBH explaining it as well (thanks to nyannyan_artist for the clip):