There could be several reasons why an ex would say they are not dating anyone while being seen dating someone else later on:
Privacy and Personal Space: Some people prefer to keep their personal life private, especially after a breakup. They might not want to disclose their new relationship status immediately to avoid any awkwardness or unwanted attention.
Trying to Move On: After a breakup, individuals might need time to heal and move on emotionally before fully committing to a new relationship. Saying they are not dating anyone at first could be a way to avoid complicating things with their ex-partner or their social circle while they are still processing the breakup.
Testing the Waters: People might want to test the new relationship before making it public. They might not be sure about the longevity of the new relationship or might want to see how compatible they are with the new partner before sharing the news with others.
Respect for the Ex-Partner: By not disclosing their new relationship status immediately, the ex might be trying to be considerate of their former partner's feelings. They might not want to hurt or upset their ex by announcing a new relationship too soon.
Fear of Reactions: Depending on the circumstances of the breakup, the ex might be hesitant to share their new relationship status due to potential negative reactions from their ex-partner or mutual friends.
Casual Dating vs. Serious Relationship: It's possible that the person is dating casually or taking things slow with the new partner, and they might not consider it an official relationship yet. Therefore, they may not see it as "dating anyone" in the strictest sense until they feel more committed.
Miscommunication or Misunderstanding: It's also possible that there was a miscommunication or misunderstanding between the ex-partners, and one party assumed that they were discussing exclusive dating while the other did not.
Regardless of the reasons, it's essential to remember that everyone handles breakups and new relationships differently. While it can be hurtful to see an ex move on, it's essential to focus on your own well-being and personal growth during this time.