Most schemes of this kind are just more complicated cookies that people hope will avoid the GDPR provisions by dint of being obfuscated. What the article is discussing looks (at first brush) to be a sensible way of aggregating users up-front before it hits the database, rather than later. So no personal data is stored. Does this meet the requirements for a site to avoid notifying users under the GDPR? I have no idea.…
Not necessarily. It's a bit complex, but the fact that Fathom ingests personal data at all likely means that they must still be disclosed by whoever is using Fathom's code.
On the other hand, if Fathom were able to push data ingestion into the first party's infrastructure so that only aggregated data hits Fathom's own infrastructure, I'm pretty sure that would put Fathom in the clear due to the fact that they are no longer processing or storing personal data.
To Fathom's credit, I am a big fan regardless of the steps they seem to be taking with anonymization, and I will consider using them instead of GA for future projects.