Ok first, other than it being stated, is there a story or short connected to the Daemon Primarch going into exile?
I believe it was the Word Bearers series which revealed the information concerning Lorgar's exile.
It is unknown why he entered the Templum Inficio or what he has been occupied with for millennia, beyond declaring that no one should disturb his meditations. But considering he is now a daemon, he seems to have removed himself from the actions of his Legion.
Is Lorgar not truly the chosen of Chaos? Reading Aurellian(sp?) he sure seems to be.
It doesn't seem that he is. At least three Primarchs had their egos massaged by Chaos by claiming they were the chosen; Horus, Lorgar and Magnus. Lorgar may have been chosen to initiate the Heresy by corrupting Horus, but his purpose (in the eyes of the gods) beyond that is unclear.