AXSelectedRows: Getting selected rows for AXOutline
I guess getting the value of AXSelectedRows is just something that has not been implemented. When I open a Finder window in list view, select two rows, and run this script:

tell application "System Events" to tell window 1 of process "Finder"
    value of attribute "AXSelectedRows" of outline 1 of scroll area 1 of splitter group 1
end tell


The result is:

{application "System Events", application "System Events"}


However you can use a whose clause to get the selected rows without using a repeat loop:

tell application "System Events" to tell process "Finder"
    rows of outline 1 of scroll area 1 of splitter group 1 of window 1 where selected is true
end tell