Tafsir of Al-Anbiya' 21:99

The apparent meaning of "entering" here is the act of ingress, and the address is directed towards the disbelievers and what they worship, by way of generalization (taghlib).

"If these were gods" (as you, the worshippers of them, claim), "they would not have entered it." Since it has been established that they have entered it in the most complete manner—in that they are the fuel of Hell—it is necessarily impossible for them to be gods. This is evident in the fact that what is meant by "what they worship" are the idols, not the devils; for the intention is to prove the contradiction of what they claim. They claim the divinity of the idols, not the divinity of the devils, such that the entry of the latter into the Fire would serve as an argument against their own divinity. Indeed, the devils who are worshipped fall under the ruling of the text by way of implication, so do not be heedless.

"And all of them"—the worshippers and the worshipped—"will be therein eternal," remaining forever.