I figured it out but this seems like an XML bug to me. The 1.10 version wouldn't open in FCP 10.6.5 (1.11 is too new for FCP 10.6.5) and neither one would open in FCP 10.7.1. This is running macOS Sonoma 14.3.
It appears to be choking on a modDate string.
Right-click on either of the XMLs and choose Show Package Contents. Open the info.fcpxml file in a text editor (I like BBEdit). Inside, look for:
<project name="XML Test" uid="83B5AD96-5D1F-4608-B247-49B2C0E8FCFB" modDate="2024-02-07 1:28:59 PM -0600">
In the modDate string, remove " PM" I'm not sure if this is necessary but I also changed the time to 24 hr like this 13:28:59
<project name="XML Test" uid="83B5AD96-5D1F-4608-B247-49B2C0E8FCFB" modDate="2024-02-07 13:28:59 -0600">
and then save. You should be able to import it now.
The 1.10 XML appeared to even have an odd space character in front of the PM.

You should submit these sample XMLs to FCP Feedback via Final Cut Pro->Provide Final Cut Pro Feedback... or use this link Feedback - Final Cut Pro - Apple and maybe include a link to this discussion.