Good news for all. Next version from nPlayer support pip but only the video format, the Quick Time supports.
Why other video format not support?
This is Apple problem. Has not right api for other video format.
How activate/use PiP in the next version?
Easy PiP in settings is on. Make sure you use video format the QuickTime supports and in playback decoder menu QuickTime. Then tap on Pop up Play. You are now in PiP mode.