Github actions unexpectedly build multi-platform image, without setting up Qemu - docker

Question

I'm trying to troubleshoot a Github actions issue, and it looks like Github and/or Docker is doing a cross-platform build for me when they shouldn't.
Using buildx, but specifying only linux/amd64 platform for the builder.
Not setting up Qemu explicitly.
Asking for linux/arm64,linux/ppc64le platforms for the image.
... and it builds! How?
I even added docker buildx inspect command, and it prints what I expect, e.g. only Intel platforms are listed.
Here's my test repo: https://github.com/haimgel/test-docker-build
Here's my action file from there:
name: TEST
on:
push:
env:
REGISTRY: ghcr.io
IMAGE_NAME: ${{ github.repository }}
jobs:
test:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout#v3
- name: Extract metadata (tags, labels) for Docker
id: meta
uses: docker/metadata-action#v4
with:
images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
- id: builder-github
uses: docker/setup-buildx-action#v2
with:
platforms: linux/amd64
- run: |
docker buildx ls
docker buildx inspect --bootstrap
- uses: docker/login-action#v2
with:
registry: ${{ env.REGISTRY }}
username: ${{ github.actor }}
password: ${{ secrets.GITHUB_TOKEN }}
- uses: docker/build-push-action#v3
with:
context: .
platforms: linux/arm64,linux/ppc64le
push: true
tags: ${{ steps.meta.outputs.tags }}
labels: ${{ steps.meta.outputs.labels }}