Ok, so I think I've found the answer and will just post here to maybe help someone with a similar problem in the future.
Basically, Google would not be showing a publication date in the SERPS that it plucked out of the sky. Knowing that, I checked my copy, my images and my entire html to see if the (wrong) date had crept in somewhere.
Viewing the source code didnt help me here, but I did click on the youtube video I have embedded on the page. Turns out the publication date for the embedded video on youtube is the (wrong) 22. March 2011. Therefore, Google is pulling the embedded video's publication date as the publication date for the entire page.
To combat this, I've updated my YOAST SEO plugin to include dates for landing pages, and included the correct publication date on all my landing pages (in the copy). I've fetched the sites again and re-submitted my sitemaps and hopefully the issue will be resolved in the morning. If it isn't, I'm going to replace the video with one published in more recent times.