Seriously, how was this kata even published?

Javascript has no Solution Setup os Sample Test Cases.

Much like C# had these items missing until a few hours ago. (08.JAN.2023 @ 03:15 GMT-3)

This assume that the class... erm... User will be well behaved to always call the right method.
Something like:

var user = new User();
user.progress = 10099;
user.incProgress(-8);

Would break the code.

Not only the Sample Tests, but the whole Solution Setup.
Is the coder suppose to divine what the kata wants to ingest?
Classes, methods, properties.. the whole shebang.

As of 07.Jan.2023 the C# has no Solution Setup or Sample Test Cases.

I fear that the C# translation of this Kata is broken beyond repair.

You misterpreted the description.
The key IS repeated throughout the message, but you don't need to make them the same length.
A little bit of modulo math.. et voíla!

Why even use fill?
It can be solved in O(n), in a single passage.

I have to agree with some of the commenters.
Yes, this is a nice short piece of code.
However, as performance and scalability goes it is, sorry to use such forceful words, a piece of crap.
It runs in O(n²), when this problem is solvable in O(n).