Having both, -nodiskptree and -diskptree is a bit ambiguous.
1) Which preceedes if both are given?
2) What is the default? And the help text: "Use a disk-based ptree implementation. Slower, but requires far less memory" seems to imply the in-mem tree is used, which is however not the case.
If disk_ptree is the default, why not drop the -diskptree option.