At this point we're stuck because the requirements aren't clear.
In the example, 12 appeared in the output before 11, but we
don't know why: there was nothing in the requirements
statement that said it should.

The only requirement is for the longer strings to come first.

So any of these results would be fine:

12|11|10|9|8|7|6|5|4|3|2|1
11|12|10|9|8|7|6|5|4|3|2|1
10|11|12|9|8|7|6|5|4|3|2|1
12|11|10|1|2|3|4|5|6|7|8|9
...

What is the simplest XPath expression to achieve that result?

As I indicated earlier, if you know => as an operator you might consider
the following simpler:

xs:restriction/xs:enumeration/@value/string()
=> sort((), function($s) { -string-length($s) })
=> string-join('|')