"How do you get people to ____" is a fundamental question in life with many approaches to answering it, ie marketing, public relations, missionary studies, theater, etc.
From what I pointed out below, this salt processing seems to have underappreciated trade offs. But most importantly, the overarching picture of nutrient availability, not just for iodine, seems to have a bigger challenge at hand worth solving than simply using salt as a vehicle for one - which spirulina would help with in terms of protein and mercury / biotoxin removal, for example.
The point is that you don't go looking for a new thing for people to consume to get them the nutrients they need, you fortify what they already eat.
And, yes, no shit. How to get other people to do something is a fundamental question in life and it just so happens that in life, public health is about getting people to do stuff that's good for their and society's health.
Nope, you could justify very specific pragmatic approaches, just like the parent did.
And, sure, there may have been some other equally good alternatives, but analysis paralysis helps no one. So, salt.
If you're running some sort of anti-malnutrition campaign that involves local cultivation of spiritual then sure, why not add iodine to it? (Assuming that this is technically feasible. Don't know whether it is or not.) But if you are trying to solve the specific problem of iodine deficiency, then I'm not sure why you would start with "first, grow a bunch of spirulina..."
Adding a small bag of sodium iodate to a big bag of sodium chloride and mixing requires no skill or special equipment and any idiot, or shall I say cretin, can do it. Whereas running a bioreactor and somehow convince people of eating the gunk that comes out of it is far more complicated and expensive.
Edit: and you'd still have to add iodine to said bioreactor, you might as well add it directly to the salt.
Depending on people deciding to eat healthy is a surefire way to fail to meet public health goals.