"Sargassum only grows well when salinity is normal and surface temperatures are normal or cooler."

This does not mean that ocean temperatures are warmer, nor that Sargassum now grows when it's warmer. It means that ocean temperatures are not warmer, and it states exactly that Sargassum grows today at normal or lower temperatures. What did they say that leads you to claim Sargassum now grows at warmer temperatures? They didn't say that anywhere. When they state the boom in blooming Sargassum will continue, they mean that the necessary temperature conditions will continue. The ocean will not grow warmer is exactly what they are saying.

The human activities specifically referenced in the report are deforestation and nutrient increases as a result of fertilizer use. Not once does the report claim ocean temperature is increasing, and across the literature there are consistent reports of ocean temperature not increasing as AGW predicts, which substantiates their point that Sargassum blooming is happening due to ocean temperature remaining normal and nutrients increasing.

Read the words, because they mean what they say, and they do not say ocean temperature is increasing, nor that Sargassum is changing the conditions under which it blooms.