Why did Subhūti not have the thought, “I am a desire-free arhat”?

Original text of the Diamond Sutra: 世尊。佛说我得无诤三昧。人中最为第一。是第一离欲阿罗汉。我不作是念。我是离欲阿罗汉。世尊。我若作是念。我得阿罗汉道。世尊则不说须菩提是乐阿兰那行者。以须菩提实无所行。而名须菩提是乐阿兰那行者。

Literal translation: In order to corroborate that an arhat has no thought of "I am an arhat," Subhūti said to the Buddha: "World-Honored One, the Buddha has said that I have attained the meditative concentration (samādhi) of non-contention, that I rank first among humans, and that I am the foremost arhat free from desire. Yet I do not have such a thought that I am an arhat free from desire. World-Honored One, if I had such a thought that I had attained the path and fruit of an arhat, the World-Honored One would not say that Subhūti is one who delights in the practice of tranquil solitude (araṇā-vihārin). Because Subhūti in reality has nothing to practice, it is said that Subhūti is one who delights in the practice of tranquil solitude."

Subhūti's other name is "Born of Emptiness," for he could constantly perceive the emptiness and stillness of the world. Because he had attained the meditative concentration of non-contention, he never contended with worldly people: if worldly people said east, it was east; if they said south, it was south; if they said west, it was west; if they said north, it was north—because whatever worldly people say is empty, and there is no need to contend over who is higher or lower, who is longer or shorter. Subhūti had no desires or thoughts whatsoever toward the world; his mind was empty, without marks, and without wishes, doing nothing at all. He was the foremost arhat free from desire among humans, yet Subhūti never had the thought "I am an arhat free from desire," much less would he proclaim himself everywhere. He did not cling to the mark of self, nor cling to the mark of "I am an arhat"; otherwise he would be a ordinary person clinging to marks, and the World-Honored One would not have said that Subhūti is one who delights in the practice of tranquility. Because Subhūti's mind was constantly empty and still, without desire or seeking, the World-Honored One praised him as one who delights in the practice of tranquility.

Ordinary people, on the contrary, are the opposite of Subhūti: whether or not there is any real basis for it, they love to boast about themselves everywhere and promote themselves, love to argue over who is superior or inferior, love to show off, love to be pursued and admired by others, love to highlight themselves—in short, they love to market themselves so that the world knows them, and thereby that "self" of theirs is satisfied. This is the mark of an ordinary person. Apart from the mark of self, ordinary people can do nothing; they constantly live within the mark of self and the four marks, and this is the spiritual sustenance of their entire lives, indispensable. Once that "self" is suppressed, it is like a fish out of water. In short, whether a person has the Way or not can be seen in actual deeds; one's bodily, verbal, and mental actions are the most revealing, best reflecting a person's level of thought and level of wisdom.