Automator workflow to split PDF
An adaptation of the code from the above mentioned thread

on run {input, parameters}
    tell application "System Events"
        set thePath to POSIX path of (container of container of (item 1 of input))
    end tell
    return thePath
end run


worked like a charm. 

So I guess, the problem as in the presentation of the folder. While the "Set Variable" action can deal with a finder folder object and preview it correctly, the split PDF action apparently can't.