The message in that CFS was pretty clear to me, it was "We aren't doing enough, and not doing anything will doom us to yet another mass extinction event." (they aren't as explicit as that but nearly all of the literature on "Phase 3" of this stuff has nearly everything dying off.

So the first thing to check is this, is that the message you heard in the linked CFS? Or did you hear a different message than what I heard?

Assuming you heard the same message, we can talk about the next place in the conversation where things may go off the rails, a comment of the form "We already know how enhance algae blooms by fertilizing the ocean." Which related to previous work on dumping ferrous material into the ocean to create an algae bloom that would capture carbon and sequester it.

And the response to that comment, the message I heard/read was (paraphrased) "How do we know what that will do in the long term? We should not make such a move without knowing the consequences of making it."

In my reading, that has been a common response to large consequential ideas such as the fertilize the ocean. So is that something you've heard as well or is the first you have heard it?

So the messages in that exchange that I've heard are

1) We understand the mechanism of phytoplankton blooms and we know how to create them, we should try that.

2) We should not try that because we don't know if creating such a bloom would generate a net positive result, and we don't know what intermediate results it might generate as well.

I combine the RFS message of "We're doomed, none of the current things people are doing to ameliorate CO2 gain in the atmosphere are working." with "We shouldn't try things if we can't predict the outcome." and come up with, "Inaction is worse than not fully understood action if inaction is leading to destruction of the world."

Now I often lose the climate deniers on that last bit. In their belief system as I understand it, it is not something we are doing that affects climate so there is no compelling call for action on human's part.

So, what part of the point did I miss?

As for "selling it" I am not sure who is being sold here.

In my response, I was making the argument against inaction, against the environmental argument of 'do no harm.' I recognize that doctor's admit that you have to poison your patient (which does great harm) using chemotherapy when they are suffering from cancer because the alternative is just watching them die. What the doctor knows is that once the risk of cancer is gone the normal processes of the body will recover the patient to a better state of health. The argument for iron fertilization is similar, which is that while it may do short term damage, by pulling the CO2 out of the air the Earth will be in a better place after its own restoration mechanisms have undone the damage.

Sadly, unlike cancer patients we can't do clinical trials on planets, we've just got the one.