The objection assumes that every oath must appeal to something greater than the speaker. That is true of many human oaths, but it is not what a divine oath means in the Quran: Allah draws attention to His signs in creation in order to confirm a truth, not because He needs their authority.
Why the objection seems forceful
The critic is right to ask what an oath accomplishes for a self-sufficient God. In ordinary human speech, someone swears by a respected authority or sacred object to make others believe him. If Allah swears by the sky, stars, the sun, or the fig, one might therefore think that He is either placing created things above Himself or using an empty rhetorical formula. The objection adds that modern astronomy has disproved a solid dome over the earth, so perhaps the "sky" is not even a real thing.
What the Quranic oath is doing
The Quran uses an oath to focus the listener on a sign and to give weight to the statement that follows. In Surah al-Tariq, the oath directs attention to the night-comer and its piercing light before affirming the reality of human accountability:
By the sky and the night-comer. And what will make you know what the night-comer is? The star of piercing brightness.
Quran 86:1-3
This is not an appeal to a higher witness. Allah also swears by created things because they are His signs, under His ownership and power. A ruler may point to a visible monument to underline a statement without treating the monument as greater than himself. The analogy is limited, since Allah has no need of support, but it shows why an oath need not mean dependence.
Indeed, the Quran sometimes has Allah swear by His own Lordship, and sometimes by features of creation. The variation serves the subject being discussed. The sun and moon suit a passage about proportion and moral direction; the fig and olive evoke recognizable signs and blessings; the night-star suits a passage about divine watchfulness and human origin. The objects are not being worshipped or exalted above their Creator.
What "sky" means here
The Arabic word sama' can refer broadly to what is above us: the visible sky, the atmosphere, or the higher celestial realms. It does not by itself mean a solid physical shell. Quran 21:32 calls the sky a "protected ceiling," describing its function and appearance from the human standpoint, not requiring the ancient image of a rigid dome.
And We made the sky a protected ceiling, yet they turn away from its signs.
Quran 21:32
Likewise, "the sky with its returning" in Quran 86:11 has been explained by classical commentators in connection with rain returning from the sky, among other observable patterns. Nothing in the verse requires Allah to swear by a nonexistent solid object. The sky, understood as the realm above us and the ordered system associated with it, plainly exists.
What this argument does and does not establish
The objection identifies a possible misunderstanding of divine oaths, but it does not show incoherence or a conflict with astronomy. It would succeed only if the Quran claimed that Allah needs created things to validate His speech, or that the sky is a solid dome. It claims neither. The oaths are invitations to look at creation as evidence of its Creator, while the truth of Allah's word depends on Allah alone.