They don’t need to, as clip-retrieval is licensed under the MIT license.

The MIT license disagrees:

  The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

Besides, even if you don't interpret this as "attribution", it's still ironic coming from this particular project.

It is ironic, but MIT license definitely does not require attribution from websites using the code on the backend (otherwise you'd see a rather large attribution list at the bottom of Google.com or any website...)