Signed xml alone is a wildly confusing idea, as the signatures get embedded as elements in the document being signed. There’s a wild set of rules on how to make xml canonical, sign, add the signature, etc. It’s nontrivial.
Then the incredibly stupid need to modify the signed document to insert the signature online so verifying it requires a full blown parser among other things
Worse this is done at an element level not a document level as noted in the linked article.
Really, it’s not that simple. It typically requires a while xml library for dealing with it that is error prone. Check the number of errors and cves for libxmlsec for example. Or even the versions in C# or Java.