While working on the HTML document in your program, you have it always represented as a tree of nodes, a nested list often. When you want to insert user input, that user input is available as a string (unless you first turn it into something else). When you put a string into that nested list, and then at the very end, when you form the response, ask SXML to turn your tree into the HTML string, SXML will error, because it wouldn't know how to handle the string.

(All the other nodes are symbols with potentially subtrees in them, which are also symbols with potentially more subtress in them.)

If you put your user input into one such node as a value of that node, lets say a paragraph tag <p>, then SXML sees the user input as it is, a string. It will make sure, that when it turns the tree into HTML, that string remains merely a string, by escaping things, so that for example when your string is <evil-tag-with attrs="bla"> this is exactly what will be seen as text inside your paragraph text, and not be interpreted.

Basically, SXML will render it like: "Oh, you want some string to be inside your p tag? Sure, can do ... This is what you wanted to be displayed to the user ... You didn't want to run that, did you? Nahh, because then you wouldn't have passed it in the SXML as text content of a p tag."

So I think the answer to your question is "Yes.".