-- Generaly reply --
Indeed, it's not true.
The length of the data that you can store in the .mark property, is a maximum of 939 characters, if your sockname is only 1 character long. The longer the sockname, the less bytes you can store, similarly with the general string too long limit, and variables. The longer the variable name, the less bytes you can store in it.
sockmark m $str(.,939)
sockmark mm $str(.,938)
sockmark mmm $str(.,937)
The reason that it states the 512 bytes limit in the help file, is because Khaled/Tjerk forgot to update it in the help file, since it states the following in the versions.txt
08/01/99 - mIRC v5.5
142./sockmark now allows storage up to the max. length of chars that mIRC
usually handles for other lines.