This is an edge-case for sure.
Here’s the source code:
print(FilePath(#"C:\foo"#).isAbsolute)
print(FilePath(#"+:\foo"#).isAbsolute)
print(FilePath(#"€:\foo"#).isAbsolute)
The three paths are valid absolute paths on Windows, however the output of this program will be true, false, false.
For more info see https://www.ryanliptak.com/blog/windows-drive-letters-are-not-limited-to-a-z/.