Good to know about GapJumpers, and in many hiring scenarios, it's a necessary legal gap what you bridge.
However, iirc the discussion wasn't around "why gapjumpers", rather, "why yet another 3rd party"?
For Mozilla People Team:
I don't think it's an easy question to answer, and Mozilla's use of GapJumpers should be opt-in for the candidated. As in, "as an applicant, if you're okay sharing your credentials with us, cool, you void your warranty to sue us for partiality of selection - if you fear there might be such case, here... go with GapJumpers for blind selection". I'm sure there are people with swing for either side (and rarely someone who is indifferent - it's a mutually exclusive, either this or that pref), hence it's better to have both the options open and not force one preference over another.
Just my under-thought 2 cents.