Right and I linked that article because it's functions as a media literacy litnus test. It takes the viewpoint of the CEO and the scientists as equally valid, and you did get the main points, but you missed the lead that was buried:

A paper he coauthored last year in Nature Communications, using the massive sargassum seaweed bloom in the Atlantic in recent years as a model, concluded that seaweed farming in the ocean could even become a source of increased carbon dioxide. That’s because the seaweed competes for nutrients with other carbon-sucking species like phytoplankton, among other complex biogeochemical feedback effects.

Which if you actually look at the paper from the scientist (and ignore the bullshit from the CEO):

Ocean afforestation at the scale of Sargassum growth in the GASB during 2018 could contribute −0.0001–0.0029 Gt CO2 of CO2 removal, if all of the seawater CO2 consumed through biomass formation is balanced by permanent influx of atmospheric CO2.

In other words, carbon source to negligible because it kills the photoplakton was already doing that, and doing it more efficiently (albeit at a lower biomass). The paper also, briefly, touches on other concerns (where we get a nice crossover with solar radiation modification) which it unfortunately doesn't delve much further into:

Furthermore, we estimate that increased ocean albedo, due to floating Sargassum, could influence climate radiative forcing more than Sargassum-CDR.

It makes climate change worse because it acts as a potential net CO2 source, requires maintenance and human intervention to maintain, destroys the local ecosystem which was doing carbon sequestration in the first place, and lowers the ocean albedo thus increasing radiative warming.

If you want to talk SRM instead the oft cited paper is this one However the final line is the important one:

The sobering reality is that unanswered questions such as these will remind the research and policy communities that relating climate response to anthropogenic perturbations is still a long way from being an exercise in engineering design.

As it was published in 1992 a lot of the questions it left at the end have answers now, and there have been attempts at some engineering design. Why don't you try to find one you think is a good potential and we can drill into its possible pros/cons (warning that meteorological stuff gets real math heavy, real quick).