I asked this question in softwareengineering.stackexchange.com but it was pointed out to me that it is better suited to be posted here. I deleted the original thread in the other subforum.
So the specific question is: Is it allowed to incorporate MIT licensed work in closed-source projects thereby practically bypassing the necessity to deliver with the new work an extra file (or link to a site or something like that) stating a copyright hint of a portion of the code which is licensed under MIT?
Actually the MIT license reads like it is not that ambiguous at all. It states
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
I found these threads on this site, most notably: https://softwareengineering.stackexchange.com/a/264709/262045
even though you legally claimed ownership of the code (as explicitly permitted by the MIT license), you should include a notice to the effect of "portions of this code were originally created by such and such" and state the date of their claim to copyright. But you don't necessarily have to do so since you've re-licensed their project and assumed liability.
but also: https://opensource.stackexchange.com/a/4716/7359 (begs the question is attributing the same as including a copyright notice?)
They read as if I can just just "legally claim ownership" and don't have to state their claim to copyright (doesn't mean I want to strip them of it which I know is not possible by law, just not stating it, the reason being to not give the competition - which is quite strong in my market - a headstart how to just come up with and sell basically the same product. I customized and added to the MIT code heavily, but still the more competitors are out there the higher possibility that at some point someone will be able to reproduce what I made).
On the other hand there are enough sources that state the (for me perceived) different conclusion, like:
But if you distribute binaries, the license text (typically) gets compiled away, therefore you are no longer fulfilling the single condition in the license. What you need to do is include the license with your binary distributions, maybe as a text file, or embedded in the program's about dialog.
Further complicating the issue, what about if the author of the MIT licensed work answers an email from me saying:
"Yes, the code is free to use (and modify). There is a LICENSE file with MIT license [..describing where to find it..]. A short acknowledge note would be appreciated (but it's not required)."
Again raising the question whether he means attribution or the absolute legal necessity to include the copyright notice.
Most confusing me is the answer by GlenH7 in the above stated thread in https://softwareengineering.stackexchange.com/a/264709/262045 when comparing it to all the evidence to the contrary. He seems credible though and as I cannot comment on his answer not having 50 rep my question is, do I understand him wrong or IS there a indeed quite glaring ambiguity to the MIT license with respect to this matter? And finally, if there is no way to not include the copyright notice under normal circumstances, does the author mailing me these words change that fact in any meaningful and legal way?