I googled it and got a bunch of articles about data breaches of Opera services.
What I understood the GP post to be is that the owners of the browser do shady stuff, which I found no evidence for on Google (or DDG). So was Opera compromised by a third party or is Opera compromised by the first party (malicious owners?).
Heads up, Mozilla is on the way to be notorious Google services/Google Cloud user [0]. Recently, they started adoption of the Google Spanner in the Firefox-Sync related backend services, while in others they adopted the Google Pub Sub. The use of GA and GTM might seem like hypocrisy but my guess mozillians don't have enough workforce and/or assets to control infrastructure to roll their own analytics platform.