There's references from the ME/CFS community testing this back in 2010. Considering how hopeless and desperate these people are and how they test everything on or off label, solo or with their doctors, if it was as simple as that, millions of people would be cured. Thats like "Have you tried yoga?"
ME/CFS sounds like it has way many more possible causes, and different "types" of it.
Whereas Long Covid is caused by a virus that more and more is linked to endothelial damage causing microcirculation problems. And Fucoidan is almost perfect for that job.