You contribute code to slint under MIT and you can also use that contributed code under MIT or any other license of your choosing, it stays your code after all. You can not use other peoples code from the slint repo under MIT though, that is correct. The royalty free license tries to get as close to MIT as we can while limiting the use on embedded... but with that limitation in place it is of course not an open source license.
Contributing back to Slint is in no way required, so if you do not like our contribution terms, then you are free to not do so. Ypu are also free to use something else if you do not like our license terms.
We try to make all of the terms as clear as possible. We rewrote the Slint licensing page several times, often with extensive community feedback, to get it as clear as it is right now. If you have ideas on how we can improve, I am all ears.
You do not even need to patch it out. It is basically an extra entry in the user db (think: /etc/passwd). Just don't fill it in and you are done, just like your location and a ton of other stuff you can put there if you want to.
Nobody can read it outside your system either... it's a defined place when (not if -- not anymore) something wants to store that information. Better have a properly protected defined place ready than have 10 different programs request that info separately and store it all over the place.