In the comments below this answer, it is stated that while there is no ShareAlike clause (that would be CC BY-NC-SA), you would not be allowed to re-license Adapted Materials under commercial terms.
Is this interpretation of this license correct?
This interpretation is correct.
The share-alike clause in other licenses is only relevant to contributions made by others than the original licensor. In fact, the CC-BY-NC 4.0 license does not grant you the right to sub-license the original work at all, as shown in section 2(a)(1) (emphasis mine):
Subject to the terms and conditions of this Public License, the Licensor hereby grants You a worldwide, royalty-free, non-sublicensable, non-exclusive, irrevocable license to exercise the Licensed Rights in the Licensed Material
Instead, every downstream recipient gets an offer to use the work under the terms of the original license by section 2(a)(5)(A):
Offer from the Licensor – Licensed Material. Every recipient of the Licensed Material automatically receives an offer from the Licensor to exercise the Licensed Rights under the terms and conditions of this Public License.
While the license does not impose any restrictions on the Adapter's License this is explicitly defined in section 1(b) (emphasis mine):
Adapter's License means the license You apply to Your Copyright and Similar Rights in Your contributions to Adapted Material in accordance with the terms and conditions of this Public License.
and so this right only extends to own work. This does not grant any additional right to use the original work without the restrictions of the original CC-BY-NC license.
So, re-licensing the original work is not granted by this license.
Someone are allowed to license their contribution to some CC-BY-NC materials under commercial terms - but they're not allowed to re-license the entire project in this way.
This was a lot more difficult than I initially imagined, and cudos to neo who made me see the light
:-). Since I am aiming for the Sportsmanship badge, and since I've spent some time on sorting this out for myself, I'll keep my (heavily rewritten) answer online.
First, we really need to determine what is licensed in the case of an adaptation. Here is the definiton (sec. 1(a)).
Adapted Material means material subject to Copyright and Similar Rights that is derived from or based upon the Licensed Material and in which the Licensed Material is translated, altered, arranged, transformed, or otherwise modified in a manner requiring permission under the Copyright and Similar Rights held by the Licensor.
We're not talking about an aggregate work here (a compilation of separate work that are easily separated and that can co-exist with different licenses for each component). Adapted Material is the result of a process of transforming materials "derived from or based upon the Licensed Material" where the "Licensed Material is translated, altered, arranged, transformed, or otherwise modified in a manner requiring permission under the Copyright and Similar Rights held by the Licensor."
This means that it may Adapted Material very hard to determine what is original and what is adapted material. If a sub-routine does not work because the original authors has the got a comparison test in it backwards, and I create a fork and fixes the bug: Is the entire fixed subroutine Adapted Material, or just the change from "
>" to "
However, when Adapted Material is created, both licenses allows you to add an Adapter's License, and they differ in what they permit.
The CC-BY-NC says (section 3(a)(4)):
If You Share Adapted Material You produce, the Adapter's License You apply must not prevent recipients of the Adapted Material from complying with this Public License.
The CC-BY-NC-SA license instead says (section 3(b)(1)):
The Adapter’s License You apply must be a Creative Commons license with the same License Elements, this version or later, or a BY-NC-SA Compatible License.
So in the case of CC-BY-NC-SA, your own improvements must also restrict users from commercial use, while in the case of CC-BY-NC, you can permit this for your own Adapted Material.
To figure out exactly what these terms apply to, we need to understand 2(a)(1) and 2(a)(5)(A). The first states that the license you receive from the original author is non-sublicensable and you instead gets an offer from the Licensor to exercise the Licensed Rights under the terms and conditions of this Public License. (My heartfelt thanks to neo for pointing this in his answer).
So you're explicitly allowed to add your own license (otherwise, it it would be meaningless to talk about an Adapter's License).
However, in the case of CC BY-NC, since the original license forbids commercial use, if your Adapter's License do not, you need (somehow) to make it clear to downstream recipients what license applies to what part of the project. You can do this by distribution your part as a patchfile - and if your contribution is substantial it may even inspire downstream recipients who wants to get rid of the NC-restriction to do a clean-room rewrite the files that makes up original.
However, this exercise mostly serves to remind us that extreme caution is required when one encounter materials that has (or at one point in their history has) been subject to the NC-clause.