Thunberg, British navigator Nikki Henderson and the Australian family who own the vessel set sail back to Europe on the 14-metre (45-foot) La Vagabonde catamaran on November 13 from Hampton, Virginia on a voyage of more than 5,500 kilometres (3,400 miles).
Thunberg herself has not responded to the new rumors, although it is probably fair to assume that she has many things on her plate, and probably does not feel too excited to participate in meaningless discussions. "We are angry and frustrated, but for good reason", she added.
Greta missed last Friday's climate strikes as heavy winds delayed her arrival to Lisbon, but she is due to join thousands of activists marching in Madrid this week on the fringes of the COP25.
She had originally planned to travel from the USA to a United Nations climate summit in Chile.
Ms Henderson tweeted: 'I chose to help @Sailing_LaVaga and support Greta because she is changing the world - simply by standing up for what she believes is right and staying true to her values.
She says she travelled by catamaran instead of plane to "send a message it is possible" to live a sustainable lifestyle.
Members of the global group called Extinction Rebellion held up a banner in Russian that read: "Climate Crisis. Climate justice", activists chanted. "To take action immediately".
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Thunberg refuses to fly because of the carbon footprint.
Some activists jumped into a nearby fountain while others threw them life jackets.
Others dressed in red robes with their faces whitened to symbolize the human species' peril danced briefly before police moved in to end the protest.
Her arrival coincided with a new scientific report saying there is mounting evidence that the world is getting ever hotter.
Levels of greenhouse gases including carbon dioxide hit record levels past year, the World Meteorological Organisation has said.
"Since the 1980s, each successive decade has been warmer than the last", the agency said.
In a report released on the sidelines of this year's United Nations climate change conference in Madrid, the agency said this continues the trend that "since the 1980s, each successive decade has been warmer than the last". The UN Environment Programme has also warned emissions would have to fall by 7.6 per cent a year up to 2030 to limit temperature rises to no more than 1.5C above pre-industrial levels.