Even if you hash somebody's full name, you can later answer the question "does this hash match the this specific full name". Being able to answer this question implies that the anonymisation process is reversible.
What they do is pretty simple. They overwrite the data fields with the text "<Anonymized>". No hashes, no identifiers, nothing. Everything is gone. Plain and simple.
That doesn't mean that hashing is enough for pure anonymity, but used properly hashes are definitely a step above something fully reversible (like encryption with a common key).
if you have an ad ID for a person, say example@example.com, and you want to deduplicate it,
if you provide them with the names, the company that buys the data can still "blend" it with data they know, if they know how the hash was generated... and effectively get back that person's email, or IP, or phone number, or at least get a good hunch that the closest match is such and such person with uncanny certainty
de-anonymization of big data is trivial in basically every case that was written by an advertising company, instead of written by a truly privacy focused business.
if it were really a non-reversible hash, it would be evenly distributed, not predictable, and basically useless for advertising, because it wouldn't preserve locality. It needs to allow for finding duplicates... so the person you give the hash to, can abuse that fact.
Additionally in the case of ipv6 it can be tied to a specific device more often. One cannot rely on ipv6 privacy extensions to sufficiently help there.
Edit:
This is a different bear:
Also, Bear claims to be GDPR compliant: https://bear.app/faq/bear-is-gdpr-compliant/