No. You're not understanding the post nor the commentaries.
In the KJV the word firmament means an expanse רְקִ֥יעַ rə-qî-a‘. It's not a capitalized proper noun like a city (Chicago), it's an expanse, an area. In the KJV, Genesis keeps referring to the sky area as a firmament/expanse or heaven. Sometimes when it reads the expanse/heaven, it's referring to the area of the moon, sun, stars, outer space. We need to use context to determine which it means.
Sometimes when it reads the expanse or heaven, it's only referring to the sky area. Example: Gen 1:20 " And God said, Let the waters bring forth abundantly the moving creature that hath life,
and fowl that may fly above the earth in the open firmament of heaven."
Do you, Romans34, believe Gen 1:20 reads the fowl are flying in outer space with the stars?
In Gen 1:6-10, when it reads firmament (KJV) it's referring to the sky area. God separated the waters between the oceans/lakes and the waters in the cloud. The expanse here is the area between the surface of the earth and the bottom of the clouds.
In fact, many versions of the Bible don't use the word firmament, it uses vault and sky.
NIV
And God said, “Let there be a vault between the waters to separate water from water.” 7 So God made the vault and separated the water under the vault from the water above it. And it was so. 8 God called the vault “sky.” And there was evening, and there was morning—the second day.
9 And God said, “Let the water under the sky be gathered to one place, and let dry ground appear.” And it was so. 10 God called the dry ground “land,” and the gathered waters he called “seas.” And God saw that it was good.
AMP
6 And God said, “Let there be an [
a]expanse [of the sky] in the midst of the waters, and let it separate the waters [below the expanse] from the waters [above the expanse].” 7 And God made the expanse [of sky] and separated the waters which were under the expanse from the waters which were above the expanse; and it was so [just as He commanded]. 8 God called the expanse [of sky] heaven. And there was evening and there was morning, a second day.
9 Then God said, “Let the waters below the heavens be gathered into one place [of standing, pooling together], and let the dry land appear”; and it was so. 10 God called the dry land earth, and the gathering of the waters He called seas; and God saw that this was good (pleasing, useful)
and He affirmed and sustained it.