As long as the link between data and user is severed, they are compliant with GDPR. [...] As long as it's not personally identifiable, it's OK.
Wrong.
In the US, data protection refers to "personally identifiable" data, so severing the link is enough. Under the GDPR, all "personal" data is protected, doesn't matter if it has a link or not to identify the person.
The test under the GDPR, will be whether a comment has any personal data in it. If it's a generic "LMAO", then leaving it anonymous might be enough; if it is a "look at me [photo attached]" or an "AITA [personal story]", then the person can ask for it to be removed, not just anonymized.