Is it appropriate to bring to an editor's attention non-ethical publishing that can't be directly proven?
Contacting the editors may not have an effect at all. They simply cannot tell whether your accusations are facts or are just made up.

If you really want to do something about this, you should rather write to their institute or university and inform them about the suspected scientific misconduct. They often have specialists nowadays who will try to resolve such issues. Be prepared to be asked for proof.

Please note, that it might be completely valid to only have these two authors: authorship is not automatically gained by implementation work but rather by significant contribution to the paper. Please check which criteria must be fulfilled in your institute or field to become an author of a paper.