I think it was designed by the founder: Tiensonqin.
thread copied from Chinese forum (Logseq Logo - 分享 - Logseq 中文社区):
This is the early stage icon meaning “log sequence”, where “log” here denotes different blocks, like a man walking. He thought this icon was a bit too complicated and ugly.
Then he simplified it.
In the Chinese forum someone also proposed other prototypes: