With "work [...] started" I'm referring to the whole dev team focussing and working on Mistlands. Not just one person, in this case mostly Richard, experimenting with ground textures (October 2021) or terrain generation (November 2021). In February 2022, they were more or less done with nailing down the concept of the Mistlands, this got announced too (along with a little concept art). In early March 2022, work fully started on the new biome, Mistlands.

I hope this gives some more insight!