By "escape sequence" it's referring to special characters that you prefix with a backslash () to represent special characters:
https://learn.microsoft.com/en-us/cpp/c-language/escape-sequences
""123""
is an escape sequence, but it's specific to verbatim string literals & not what that sentence was referring to.
Note: the first example prints the verbatim string "\t" whereas the second prints the character that that escape sequence represents (tab):
string value1 = @"\t";
string value2 = "\t";
Console.WriteLine(value1);
Console.WriteLine(value2);
If you want to avoid using a verbatim string you could escape using backslashes:
string xml2 = "<customer id=\"123\"></customer>";
To avoid adding a backslash before every literal double quote you could also use raw string literals:
string xml2 = """<customer id="123"></customer>""";
Console.WriteLine(xml2);
See here: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/raw-string