Monzo has recently started to share their data with Equifax (our data provider).
This means that you’ll receive an alert in your upcoming changes informing you an account has been added.
Although your Monzo account might be old, because this data has just started being shared, it’s considered new information and that’s why you’re receiving the alert.
You can learn more about this by reading Monzo's blog.
Also if want to learn more about how long information stays on your report read this article.