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.