I read that same story a long time ago, but apparently it had things mixed up and this is the way it actually went down: https://www.moomin.com/en/blog/the-story-of-moomintrolls/

"On a summer day, she was discussing literary philosophy with her brother Per Olov Jansson next to the outhouse at their summer cottage in the archipelago. Tove quoted Immanuel Kant, who Per Olov immediately downplayed. To get back at her brother, Tove drew the ugliest creature she could imagine on the outhouse wall. That drawing, out of chance, is the first glimpse of a Moomin-like figure, although Tove called it a Snork."