Hi Tom and thank you for your message.
Indeed, MNX is under construction by the W3C Music Notation Community Group. However, this format will not be operational before at least one year – plus the time that will be required for apps / software to adapt to it.
Be reassured: MusicXML will keep being the norm for a long time and when MNX will be there, Newzik will be among the first companies to adopt it!
The Newzik Team