Combin very dense polygons in low zoom

I have make hillshade by 30m DEM, and it's very dense polygons,like this:

I try to make vector tiles use this:
tippecanoe -aD -aS -Z0 -z6 -L hillshade:test.geojson -o hillshade.mbtiles
The result I am expecting is like this:

But i get this:

It doesn't look like the merging is enough at low zoom(1-6)...blow is my test data:
test.geojson.zip

This is my test style:
{
"layout": {},
"maxzoom": 16,
"type": "fill",
"source": "composite",
"id": "hillshade",
"paint": {
"fill-color": [
"match",
[
"get",
"class"
],
"shadow",
"hsl(56, 36%, 21%)",
"hsl(35, 14%, 100%)"
],
"fill-opacity": [
"interpolate",
[
"linear"
],
[
"zoom"
],
14,
[
"match",
[
"get",
"level"
],
[
2,
1
],
0.06,
[
4,
3
],
0.05,
0.12
],
16,
0
],
"fill-antialias": false
},
"source-layer": "hillshade"
}

So what's the problem?

Reproduce the reported command with the attached test.geojson and compare the generated hillshade.mbtiles at zoom levels 1–6 with the expected image. Start by tracing low-zoom polygon simplification and merging behavior; done means dense polygons are combined enough to produce the expected low-zoom appearance without breaking the provided style.