Kathina robe offering ceremony and Nāgitāpadāna: a myth?

It was mentioned in the Nagita Apadana, the Kathina dana (offering) is the highest meritorious deed among the act of wholesome deeds in the realm of Sensual world (Kamavacara Kusalakamma).

Ven. Nagita pointed out: “I have never experienced in the lower realms in the period of thirty aeons with the advantage and result of offering the Kathina robe to the Noble Community of monks.” “Each and every place of my birth alone with all the worldly luxurious material gained I became a very noble superior person with the result and merits of offering Kathina robe to the Noble Community of monks.” He further pointed out that “I have never known in any lower realms excepts being born only to the human and heavenly world with the results and merits of offering the Kathina robe to the Noble Community of monks.”
Kathina (Robe offering Ceremony) at IBC

Venerable Nagitha Thera had declared that he had enjoyed heavenly pleasures for more than 100 times borne as a sakra or the king of the heaven and being borne more than 1000 times as universal king (Cakkavati Raja in Pali and Cakravarti in Sanskrit) as a result of offering one Kathina robe. THE SIGNIFICANCE OF KATHINA ROBE OFFERING CEREMONY

Above qouted explainations are from Nāgitāpadāna: a story of Ven. Nāgitā which cannot be found in tipitaka, commentaries or sub-commentaries. However, nowadays devotees pay higher attention to kathina robe offering ceremony, and offer so many things along with the kathina cīvara. This attention is won because of above story, where people are more eager to carry out highest meritorious deed. I heard some venerables explain merits of kathina in a way that says one who offer a kathina cīvara never be born again in a lower-human realm ever which could be achieved only by attaining sotāpattipala according to EBT.

What could be the reason to the origin of above story? Is it a myth? Any details on the origination of the story?
Is it ethycal to use such a story to explain merits?