I have run into this article.
This seems to be in line with this private usage of NEM technology.
The thing is that also other chains can be used privately and not NEM exclusively.
What are or what will be the user cases for the public NEM chain then?!
Who will be willing to use it and for what?! 
For transportation there are driveways, small roads, highways, trains, airports, etc. They all compliment each other. Same will happen with DLT.
Just as there are networks of networks making the internet. There will become ledgers of ledgers for a variety of data sets. Each layer providing different features. I don’t think things will evolve as simple as public / private chain. But layers of private chains communicating with layers of public chains.
Right now businesses keep their data on central servers, and have to pay for ongoing upkeep, security, redundancy, insurance, etc - and is more susceptible to human error & malicious intent.
Some of this data also needs to be available to third parties / customers / clients / etc. This is where a public gateway in necessary. Instead of giving the 3rd party access to a data set within the central network, that data can be encrypted and put into an additional layer of which the individual client has the keys to. - Much like your current credentials grants access to your existing accounts.
Lastly, these individuals will need the means to share this data with additional parties. EG. You request your bank statement, which is encrypted and published to a public address you have keys to, from the banks private chain. You then forward these statements to your accountant, who then logs them within the office private chain, completes your tax prep, and again encrypts and publishes them to a government address on public chain. And once again, they store that data in their private chain.
And even within this quick example there can be a multitude of other “things” happening, such as zero knowledge proofs, onion routing, atomic swapping, multi signatory, aggregated transactions, non fungible asset transfer, etc.
We’ve barely begun to scratch the surface of what is possible.
Long story short, NEM needs to remain competitive for both public and private chain applications, as well as remain open minded and adaptive.
Pubplic chains bring transparancy, resiliency and depending on what you’re doing also infrastructure that you don’t need to build yourself. If any of those are desirable for a project then they’ll build on a public chain.
Luxtag is a good example. If Luxtag built on a private-chain the whole selling-point would be come absurd to me and frankly they shouldn’t even use blockchian tech at that point. In this case the transparancy aspect is at play.
Connecting private chains via one public chain may also become a thing at some point. In this case the infrastructure aspect comes into play.
I don’t think there’s a big resiliency aregument for nem but eth and e.g. storage solutions like sia would be good examples for this.
For better or worse, most enterprises will require high throughput and privacy thogh, which will require private networks or DLT other than blockchains.