After reading Curze`s beck I had the impression, that the Emperor, after communicated closely with newfound Primarch, upon arrival to Terra just grabbed the first available son, saying things like, “Konrad, is that your brother Fulgrim. Fulgrim - it’s your brother Konrad. Make a man out of him “- and flew off at top speed to the other side of the galaxy.
I think glamorous nyashka Fulgrim has been a little stunned by such a surprise)))
( More detailed version of this drawing - fav.me/d6njbyh