That's not a comment, that's an update to the answer. You have 3 options, which in descending order of priority should be:
- If you know, through your own knowledge, that the addition is correct, approve it.
- If you don't know whether the addition is correct, skip it.
- If you fear that the addition may be incorrect, or believe that this is more of a comment than an update, reject it to stop it being wantonly approved.
Simply put, if you don't understand the subject matter and the edit appears to be a, potentially correct, update to the answer you should skip the edit. You do not have enough domain knowledge in order to do anything else.
Realistically, this doesn't always happen and I understand that some people might reject. If this is the case you should reject with a custom reason explaining to the editor that they should comment under the answer and ask the poster to update. If the editor can not comment then you can do it for them, link to the suggested edit, which should have notified the original poster anyway.
Don't forget that Stack Exchange's goal is to help people get good answers to their questions and every post is editable by everyone in order to attempt to achieve this.