They've known for a while now, and been talking about it for well over a year.
On Jan 1 2020 a new IMO (International Maratime Organization) regulation went into effect. The shipping industry drastically lowered the sulfur content of its fuels and the SOx content of ship exhaust plumes dropped by about 77%. (Other aspects of the fuel change also reduced some particulate pollution, too.)
The COVID sequestration also reduced shipping (and cloud-seeding exhaust from it), along with aircraft contrails and upper-atmosphere dust, and dust-generating industrial processes and transportation activity, which (like volcanic dust) also reflect sunlight over the ocean and lower temperatures.
I've seen claims that the reduction in ship exhaust plumes, alone, are enough to account for ALL the sea temperature rise since 2020, and that with the low-sulfur fuel in continued use the bulk of that excess heating will continue even as activity ramps up post-COVID.