You are right! It is a Mahāyāna sutra that was written in Sanskrit (not a Pali sutta).
About three hundred years after the passing away (Parinibbana) of the Buddha, Mahāyāna emerged in India, when they started writing their own sutras.
One way to check whether it is a sutra or a sutta is look for Sanskrit words in the sutras. For example, the one you quoted starts with:
“Thus I have heard, at one time, the Buddha dwelt at Shravasti, in the Jeta Grove, in the Garden of the Benefactor of Orphans and the Solitary, together with a gathering of great Bhikshus, twelve hundred fifty in all and with all of the Bodhisattvas,..”
The Pali words for those highlighted Sanskrit words are: Savatti, bhikkhus, and Bodhisattas.
Some other prominent Sanskrit words are: dharma (for dhamma), karma (kamma), nirvana (Nibbana).
So, it is not hard to distinguish between Pali suttas of Theravada and Sanskrit sutras of Mahāyāna.
This and other historical facts are discussed in the section: “Historical Background“.