<p>I suppose there could be historical reasons for this naming and that other languages have similar feature, but it also seems to me that parameters always had a name in C#. Arguments are the unnamed ones. Or is there a particular reason why this terminology was chosen?</p>

Yes, you're absolutely right (to my mind, anyway). Ironically, although I'm usually picky about these terms, I still use "parameter passing" when I should probably talk about "argument passing". I suppose one could argue that prior to C# 4.0, if you're calling a method you don't care about the parameter names, whereas the names become part of the significant metadata when you can specify them on the arguments as well.
I agree that it makes a difference, and that terminology is important.
"Optional parameters" is definitely okay though - that's adding metadata to the parameter when you couldn't do so before :) (Having said that, it's not going to be optional in terms of the generated IL...)
Would you like me to ask the team for their feedback?