Tafsir of Ya seen 36:15

(They said) — meaning the people of the town, addressing the three — (You are not but humans like us) — without any distinction for you over us that would necessitate your being singled out for what you claim. (Humans) is in the nominative case because the negation is nullified by illā (except). It acts in the manner of laysa (to be), and when its negation is nullified by the entry of illā upon the predicate, its resemblance to laysa weakens, and thus its operation becomes void, contrary to the view of Yūnus. Mithlunā (like us) is an adjective for bashar (humans), and it does not gain definiteness by the genitive construction, as is known in grammar.

(And the Most Merciful has not sent down anything) — from what you claim is revelation — (upon anyone). The outward appearance of this statement implies their acknowledgment of Divinity, yet they deny the Messengership and seek intercession through idols. The specification of this Exalted Name from among His Names—Mighty and Majestic is He—was due to their claim that Mercy refuses to send down revelation, for it necessitates a burden from which no benefit returns to Him—Exalted be He—nor does the attainment of His reward for the servant depend upon it.

It is also said that the mention of "The Most Merciful" occurs in the narration, not in the original speech, and that they actually said, "There is no god and no message," based on some traditions suggesting they said, "Is there a god for us other than our gods?" The expression used here reflects His forbearance—Exalted be He—toward them and His Mercy in not hastening the punishment at the moment of their denial. Perhaps the former interpretation is more primary and evident, and there is no certainty regarding the authenticity of the tradition that contradicts it.

(You are not but lying) — regarding what you claim. This is an explicit declaration of what they intended by the two previous sentences. The choice of takdhibūn (you are lying [as a continuous/renewed action]) over kādhibūn (you are liars) is to indicate renewal.