GDPR Art 4.(1) 'personal data' means any information relating to an identified or identifiable natural person ('data subject'); an identifiable natural person is one who can be identified, directly or indirectly, in particular by reference to an identifier such as a name, an identification number, location data, an online identifier or to one or more factors specific to the physical, physiological, genetic, mental, economic, cultural or social identity of that natural person;

Posts in the Lemmy instances contain information relating to an identifiable natural person (by their user handle), as they contain the person’s ideas and opinions. Therefore the Lemmy instances are handling personal data and must comply with the GDPR.