I haven't read the paper yet but I will gamble a response to the answer to your question "Is he missing something." Then I'll have a look and see how well my response holds up.
The gist is: It doesn't look to me as if they are arguing that we actually live in a void, or that the Copernican assumption is wrong.
Sounds to me as if they are proposing a nice way to further test the Copernican notion---by raising a question which can be answered by a new bunch of Supernova observations.
The Copernican assumption (that we live in a fairly typical neighborhood) has been tested in the past and has been repeatedly confirmed. In a way, you do a scientific principle a favor by explicitly putting it at risk, because only by being tested can it be confirmed. The more times something is tested (and passes) the more confidence people tend to have in it.
So I would hope that they are not missing anything in their analysis and that the continuing SN IA observations ARE a good test.
But wouldn't it be exciting if it could be determined that we DO live in an unusual neighborhood!
Here's background on Pedro Ferreira. He has 88 papers on arxiv.
He has published a lot, peer-review. He has co-authored with prominent people.
Here's some background on Tim Clifton. He's young evidently. He already has 18 papers on arxiv.
He has co-authored with prominent people (John Barrow, Steve Shenker, Joel Weinberg)
He has a good publication record for someone at his stage.
Here is the full link to the preprint you brought up, for the convenience of anyone who wants to take a look. I shall, in a moment.