They say why in the rest of the comment.

Because everyone on the planet going vegan tomorrow will not fix the root cause of climate change which is burning fossil fuels. Three quarters of Greenhouse Gas emissions are caused by burning fossil fuels in sectors other than agriculture (in general, not just farming):

https://ourworldindata.org/ghg-emissions-by-sector

Namely energy use in industry and buildings, and transportation.

Even taking into account the power used for transport, storage etc, of food, the food system as a whole accounts for one quarter of emissions:

https://ourworldindata.org/food-ghg-emissions

So even if we stopped eating any food at all tomorrow, we'd still not be stopping climate change, which is caused ... say it with me ... by burning fossil fuels.

  Burning fossil fuels. <-. 
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                    This!

That thing right there is the cause of climate change and that's the thing what we have to address, and address immediately.