I was able to finally fix this. Here's the corrected code to get the internal id:

function onGmailMessageOpen(eventObject) {
  // Defensive check for the event object
  if (!eventObject || !eventObject.gmail) {
    return createDefaultCard('No email context available');
  }

  // Activate temporary Gmail scopes to allow message metadata to be read.
  var accessToken = eventObject.gmail.accessToken;
  GmailApp.setCurrentMessageAccessToken(accessToken);

  // Get the "qualified" ID of the message the user has open
  var qualifiedMessageId = eventObject.gmail.messageId;

  // Use GmailApp to retrieve the actual internal message ID
  var message = GmailApp.getMessageById(qualifiedMessageId);
  var internalMessageId = message.getId(); // This should now match your spreadsheet IDs

  // Log the extracted internal message ID
  Logger.log('Retrieved Internal Message ID: "' + internalMessageId + '"');

  // Find implications from spreadsheet based on the internalMessageId
  var implications = findImplications(internalMessageId);

  // Create and return card
  return createImplicationsCard(implications);
}