I might be wrong but I vaguely remember that something like “all” of the popular open source SAML libraries in the wild were vulnerable due to DSIG mishandling circa 2012. They’d blindly accept any document as valid as long as there was a signature in place that signed some element, but didn’t bother to check that that element was the set of assertions actually being used.
There were similar issues more recently (2018/19) with custom XML entities or comments. The XML signature was unaffected, but if code used the equivalent of element.children[0] to get the contents, it was possible for attackers to truncate the attribute values seen by the service provider library.
You can defend DSIG in a lot of different ways. I'll disagree with all of them and feel like I could probably win the argument; DSIG is a cursed standard. But that's not the bar you have to clear in this discussion; here you have to establish that DSIG is good for SAML. It clearly isn't.