Hello @crw17 - you are correct. Currently there isn't a way to apply a filter across multiple (or all) discussion boards within a course to see all posts from a particular student.
I say this knowing that there is a search field at the top of the discussion list page which gives a visual indication we are able to 'search by title or author'. But alas, this does not currently function across different discussions, it works only within a single discussion.
If that capability is important, may I suggest posting the suggestion as an idea? I was going to include the link to the idea suggestion page but cannot find it this morning. I believe the ideas process is to be reworked and that may well be in process. But here is a link to the current (old?) method to share an idea to improve Canvas:
https://community.canvaslms.com/t5/Instructure-Community-Guide/How-do-I-create-a-new-idea-in-the-Ins...
Best, Jeff