I think you are confusing 'ison' with 'ischan'.
No. That was taken directly from the help file.
/help ischan
isvoice nickname v1 has a voice on channel v2
isreg nickname v1 is a normal nick on channel v2
ischan if v1 is a channel which you are on. isban if v1 is a banned address in internal ban list on channel v2
Part the channel #mIRC if you're on it.
//if (#mIRC ischan) echo -a $true
Re-join it and again:
//if (#mIRC ischan) echo -a $true
