Getting value of non-existant cvars in menu in ioquake3 will be 0. Flipping the logic for fs_pure allows changing sv_pure in the menu when running on unmodified ioquake3 engine (which lacks fs_pure/fs_unpure so they are treated as 0).
This breaks compatiblity with older Q3RallySA UI VM. Though I recently broke compatibility with older Q3RallySA CGame/Game VMs to restore ioq3 compatibility anyway (removed asin/cos system calls).