I’ve edited your code for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.
You can also use the “preformatted text” tool in the editor (</>) to add backticks around text.
If you are saying that spreadsheetFunctions[fn.toLowerCase()] resolves to the name of a function then you are correct. And then of course you call a function using parentheses, passing in any arguments to the function inside of the parentheses.