No, they're not.

C# Completions 1900

When reporting an issue, be as detailed as possible. What is broken? What's your code?

No, and it's been explained several times before.

...and it won't be one of the other given digits in the expression.

The tests are ok. If you need to hardcode them, then your code is wrong.

This solution mutates the input, which could cause unexpected side-effects for the caller.

It looks like you're not using the Morse code dictionary provided with this kata, but are using one of your own. Stop it.

It is not actually brute force. It's O(n). Brute force here would be O(2^n) aka exponential.
And if you run your solution and his, you'll see, they both take around 50ms