Upaya or skillful means is a teaching which might not be literally true, but which nonetheless helps someone come to a realization of the Ultimate Truth. Skillful means is also referred to as provisional truth:
In the Lotus Sutra, the historical Buddha Shakyamuni says his enlightenment is so far beyond our understanding, that he can only communicate it through similes and parables, various forms of upaya or skillful means.
It doesn’t matter whether or not Amida Buddha is a historical being, if what he symbolizes (as a upaya) is the Ultimate Truth itself. What matters is that Dharma-body, that which Amida Buddha signifies, is a true reality.
However, the source of skillful means does matter, since only an enlightened being such as the historical Buddha is qualified to know which provisional teachings will lead others to the Ultimate Truth of enlightenment.
Amida Buddha, as a symbol of the Dharmakaya, would be meaningless if there wasn’t the historical Shakyamuni in the first place, who experienced the Dharmakaya for himself, and then symbolized it as Amida Buddha.*
In the Nembutsu, the name of Amida Buddha, Namu-Amida-Butsu, we are led by Dharma-body to the Pure Land, the realm of Nirvana. The heaven-like language used to describe the Pure Land is also a upaya for Nirvana itself.
*If the historical Buddha didn’t teach about Amida Buddha, then what matters is that enlightened teachers who came after Shakyamuni taught about Amida.
The concept of upaya in the Lotus Sutra is little different from the Pali concept of Buddhist teaching as a provisional raft to the other shore of Nirvana.