How to parse a list using simple html dom

find('text') is what you're after. It returns all text blocks found in the source.

Based on your example here's a working code:

// Test data
$input = <<<_DATA_
    <li id=xyz>
      John Johnson
    <sup>1<sup>
    ","
    </li>
_DATA_;

//Create a DOM object
$html = new simple_html_dom();
// Load HTML from a string
$html->load($input);

// >> Long answer
echo "Long answer:<br/>";

// Search all text nodes inside the target node
$search = $html->find('li#xyz text');

// Loop through each node and print it
foreach( $search as $i => $txt ) {
    // No need to specify "->plaintext" since the content is already in plain text here
    echo "$i => " . $txt->plaintext . "<br/>";
}

// >> Short answer
echo "<hr>";
echo "Short answer:<br/>";

// Specifying the index (0th here) returns the Nth element from the array containing all search results
echo $html->find('li#xyz text', 0)->plaintext;

// Clear DOM object
$html->clear();
unset($html);

OUTPUT:

Long answer:
0 => John Johnson 
1 => 1
2 => "," 
3 => 
-------------------
Short answer:
John Johnson

For more details check the Manual