Most of the forsaken seem to think that no one in the current age is as skilled or as clever or smart as they are so its no suprise that Demandred would think that anyone who can match him had to be from the AoL. That he thought Lan was Asmodean is a mystery thought. I seem to recall scenes when Rand saw Asmodean with a sword and he was not impressed. In particular when he returned after balefiring Rahvin he noted that Asmodean was holding the power weakly and a sword and he was looking around nervously. One would have thought that if Asmodean had any skill with a blade Rand who had some considerable training himself at this stage would have noticed the way he held the sword, the way he stood. Instead Asmodean came across and a man who barely knew one end from the other but forced to use it as he was partially shielded.

Asmodean did say that the forsaken were not close friends but Demandred comes across as super compitent and I am sure that he would have made sure to get a briefing on any of the current top ranking chosen, including Asmodean.

Then again Demandred was showing signs that his obsession with LTT was getting to his brain.