If I recall correctly, isowner is for IRCX servers.
~ Edit ~
From versions.txt
23/09/99 - mIRC v5.61
Changes:
..
2.Improved support for IRCX servers:
Added property $nick().owner, isowner if comparison, and "q"
property to $nick(#,N,aohvrq,aohvrq) where "q" is for owners.