Again, you are putting words in my mouth. I didn't make any comment with regard to whether the person you are learning from is an instructor or not. I said you CAN learn without formal instruction that produces a C card. You could still be learning from an instructor. I have taught a dive buddy or two how to dive a wreck (non--penetration) (as best I can, and I am a SDI Wreck instructor) without making a formal class of it or issuing them a C card. I personally know a person or two, who are not instructors, that I would trust to teach someone how to dive wrecks (non-penetration), also. If you want to get technical, I'd call it mentoring, and I think it's a good way to learn a lot of things in scuba.
I learned how to dive from a boat by going diving on a boat and getting the safety and dive briefings from a mate (a Divemaster, I think), not an instructor. I didn't take a Boat Diver class or get a C card. And I'm perfectly fine with having learned how to dive from a boat that way - for myself and for anyone else.
I learned how to drive a DPV from a guy who is not an instructor. I have yet to take the formal DPV Diver class and have no DPV cert card. I did my due diligence, though, and read what the DPV course standards prescribe for skills and asked my mentor about the "proper" way to do some of those things. I could be wrong, but I *think* I am a competent and safe (non-cave!) DPV driver now.
I guess the direct answer to your question is Yes. I'm okay with taking instruction "in scuba diving" from someone who is not certified as a scuba instructor. If you are not, that is your choice. Personally, I try to learn from anyone that can teach me anything - Instructor C card in their wallet or not. Before I actually became an instructor and started teaching, I might have been pretty inclined to presume anyone with an instructor card is actually qualified and competent to teach whatever their Instructor card says. I didn't know better, then. Now, I'll just say that, in the Venn diagram of "has Instructor card to teach X" and "qualified and competent to teach X", my experience is that the two circles overlap, but neither is a proper subset of the other.
Circling back to the start of this subplot between you and I, I repeat myself: Even if the two divers who passed away in this accident did not have full Deep specialty cards, I would not simply ASSUME that they were unqualified to be doing the dive they were doing.