Allright guys, I have figured out that I can get the product date by using getData instead of getAttributeText.
<?= $productdate = $_item->getProduct()->getData('productdate') ?>
This leads me to the next problem the date gets printed like: 2019-10-10 00:00:00. But the Magento system datapicker provides no option to set the time (eventhough the Magento manual says otherwise). This seems to be a known bug.
So does anybody know either how to fix the date/time picker or otherwise how to print the date so it doesn’t print time?