This is based on little kernel from recollection (noted somewhere in the docs)
Fuchsia's kernel (Zircon) is based on LK [0] [1] ("Little Kernel"), it's not a from scratch thing. Would be interesting to know when LK started, the first commit[2] in the git repo is a big code drop so doesn't tell the story.
[0] https://fuchsia.dev/fuchsia-src/concepts/kernel/zx_and_lk [1] https://github.com/littlekernel/lk [2] https://github.com/littlekernel/lk/commit/1d0df6996457273367...
The docs refer to 'LK' (Little Kernel) as an inspiration for the lower levels of the design. It's another project started by Travis Geiselbrecht of NewOS fame.