I'm so sorry to hear this. While we still do not have confirmation that it was kelp that led to this tragedy, we have enough information to make a reasonable assumption that kelp played at least a role in it.
I live in SoCal, and I too learned to dive in kelp. In fact, the vast majority of my dives involve at least some contact with kelp - and some of them (especially during lobster season!) are spent almost entirely in kelp. Kelp diving, while definitely manageable if you know how, is not something to trifle with. People with no experience diving in kelp should be given instructions on how to deal with it before they dive it. Kelp is one of the main reasons why California diving, while among some of the most beautiful and rewarding on the planet, is also among the most dangerous.
I will take this opportunity to urge anyone who is NOT used to California diving, and plans on diving out here, no matter how experienced you are, to do some research and learn about kelp before you dive in it. There are some good online resources for learning about diving in kelp. I'd also encourage you to talk to a DM or instructor about what to do should you get caught in it. And, the MOST important thing is: be aware that even if you do get seemingly "hopelessly" entangled in it, you are not - you CAN get out, and it's really not all that hard. Just knowing that can prevent panic, and get you to start thinking about next steps.
I've gotten myself completely wrapped up in kelp to the point where it felt like I was getting strangled by rubber bands - my own fault for rashly going after a bug where I shouldn't have. But I carry two knives, and know that no matter how twisted up, I can always get out of it. And I always have!
Here are just a few links to give some basic information about kelp diving. Spend some time googling, and you can find lots more.
Diving in Kelp | Scuba Diving Magazine
Diving 102: California Kelp Diving and the Kelp Crawl
Tips for Kelp Diving - Divester
Thoughts and prayers to the lost diver and his friends/family. Maybe this tragedy will get more new-to-CA divers to learn about kelp before they dive in it - and save one more diver from panicking and losing his/her life.
Editing to add: I do not, in any way, mean to suggest that is what happened to the lost diver in SD, or that his buddies and/or DM or boat captain did not adequately prepare him for diving in kelp.