It is two seperate $chan meanings, the $chan on its own is a remote (event) identifier, and is the channel the event occured in.
The $chan(channel name here).prop is a Widnow identifier and gives info about a channel, so
$chan( $chan ).topic, can only be isssued from inside an event or an alias called from an event, sicne it relys on the $chan remote identifier to tell it what channel to look up in the window identifier $chan()
try typing this on the mirc line
//echo -a $chan(insert a channel name here your on).topic
also see
/help $chan
both topics