I think that would be the issue - but interestingly what about actually advertising job vacancies? - you could almost 'pre-select' people by the newspaper/outlet you decided to advertise in? An advert in a Catholic newspaper or a gay magazine is going to generally attract a certain applicant in the first place...
There is no legal requirement to advertise vacancies AFAIK but when they are advertised then they cannot be discriminatory. JJ has employment vacancies on their website. They link to a job application form that asks if the person is a JW. That is discriminatory in itself.
I don't know enough about JJ to know if they employ non-Witnesses but all the stuff I have ever heard is that they don't. I don't know if they interview non-Witness applicants.
They may interview non Witnesses for the sake of procedure but even if there is active discrimination on the basis of religion then the problem is proving it. It's one thing to go to tribunal if dismissed from a job but if you never got it in the first place...