That's not what they said at all. What they said was that these blooms are not happening because of increased water temperatures...so they do believe that the water is warmer, that warmer water is not hindering the ability of the blooms, which they would expect but rather it is caused by increased run off of fertilizers and deforestation though this is hypothesis and more study is needed, a crucial point you left out.

They think that discharged nutrients may have increased in recent years due to increased deforestation and fertilizer use, though Hu noted that the evidence for nutrient enrichment is preliminary and based on limited available data, and the team needs more research to confirm this hypothesis. In addition, Sargassum
only grows well when salinity is normal and surface temperatures are normal or cooler.

So in essence the warmer water is not hindering the growth like it should be nor is it contributing to the growth so something else is attributing to it therefore these blooms are here to stay regardless of warmer water temperatures.

“This is all ultimately related to climate change, as climate affects precipitation and ocean circulation and even human activities [that can lead to Sargassum blooms], but what we’ve shown is that these blooms do not occur because of increased water temperature,” Hu said. “They are probably here to stay.”

You see they even emphasized the fact that they have proven the blooms are not caused by warmer water but by human activities [that can lead to Sargassum blooms], something they could not prove unless the water was warmer.

I am not a climate change believer but this was a rather Duh moment for you.