I believe the issue isn't specifically with anonymised IP addresses in GA. The problem is that the Google Analytics code is loaded from a third party server and, to do that, this server gets your IP address even if the data sent by the GA code itself contains an anonymised one.

I think it's personal data because you can track a visitor across multiple visits. Based on that identifier you can connect all the other data points from a visitor. I think that's not privacy-friendly at all.

I guess it depends what "anonymised IP address" means. If it's still possible to correlate data from visitors across different websites, they might as well log IP addresses directly.

The court disagreed on the basis that US federal law enforcement could force Google to stop anonymizing IP addresses at any moment, not that their IP anonymization[0] is inadequate.

0: https://support.google.com/analytics/answer/2763052?hl=en