Generally speaking when data is deleted for a context, all comments are deleted.
However comments are stored with:

1. contextid
2. component
3. area
4. itemid

These are already used for the export, but only the contextid is used in the deletion.

Although, in most cases, when deleting for one context you'll delete all comments, this must not be assumed. There can be legitimate reasons to keep data from comments.

One example:
The tool_privacy tool may allow for comments relating to a delete request. These comments form part of the response and there is a strong business reason not to delete the comments.

We must update the comments provider to take more than just contextid for deletion.