Eventually your occupational forces failed to capture documentation for this missile design.
Check if "New Tech From Coquest" is on for your campaign, try to make more occupations or capture ship/station, that have those missiles in magazines, with boarding combat.
The campaign is gone with the last patch, however I am pretty sure the option "new tech from conquest" was enabled.
Also, I had conquest a second planet from the same aliens with same missles, however even then I did not manage to dig up any documentation about them.
The only possibility was the bording combat, but I did not try it.
I think
Steve is the only one who can confirm if this is just for cosmetics purposes or there's a way to make it works.
