Yoshiro Tamura has a curious interpretation of the Lotus Sutra’s Chapter 16 in his book, “Introduction to the Lotus Sutra.”

Tamura writes:

Chapter 16, “The Lifetime of the Tathagata,” as it has been traditionally understood, reveals the eternal life of Shakyamuni Buddha. But the occasion for doing this was provided by a question raised in the previous chapter: How could the innumerable bodhisattvas who have emerged from below the earth have been taught and led to the Buddha way over the short span of the Buddha’s life? The answer is given that the innumerable bodhisattvas have been authentic disciples of Shakyamuni Buddha. That is, in view of the fact that Shakyamuni Buddha only recently became awakened and became a buddha, how could he have so many disciples? Chapter 16 reveals that in reality Shakyamuni Buddha became a buddha an infinitely long time ago, thus an infinite amount of time has passed since he became Buddha. In this way we can see that the advocacy of the Buddha’s universality is related to bodhisattvas.

The important thing to notice in chapter 16 is the way in which the Buddha’s everlasting life is revealed. The text says:

Thus, since I became Buddha, a very long time has passed, a lifetime of innumerable countless eons of constantly living here and never entering extinction. The time that I have devoted to walking the bodhisattva way is not finished even now, but will be twice as many eons as have already passed.

This means that the eternal life of the Buddha is shown through infinite, never-ending bodhisattva practice. The everlasting or eternal life is realized by endlessly doing bodhisattva practice in this actual world.

Yoshiro Tamura, "Introduction to the Lotus Sutra", p52-53

Tamura’s quote from Chapter 16 has a note indicating that it comes from Reeve’s 2008 English translation of the Lotus Sutra. However, it is not a direct quote from Reeve’s translation, which states:

Thus, since I became Buddha a very long time has passed, a lifetime of innumerable countless eons of constantly living here and never entering extinction. Good sons, from the beginning I have practiced the bodhisattva way, and that life is not yet finished, but will be twice as long as what has already passed.

I stumbled over Tamura’s quote because this is not how Senchu Murano translates this portion of Chapter 16. Murano offers:

“Good men! The duration of my life, which I obtained by the practice of the way of Bodhisattvas, has not yet expired. It is twice as long as the length of time as previously stated.

Clearly, the Bodhisattva practices preceded Śākyamuni’s enlightenment. There’s no suggestion that he continued these Bodhisattva practices.

Leon Hurvitz’s 2009 translation is even clearer on this point:

O good men ! The life-span I achieved in my former treading of the bodhisattva path even now is not exhausted, for it is twice the above number.

None of the other English translations of Kumārajīva’s Chinese Lotus Sutra suggests an unceasing practice of the bodhisattva way by the Buddha.

The BDK English Tripiṭaka translation offers:

O sons of a virtuous family! The lifespan that I first attained through practicing the bodhisattva path has not yet expired. It is twice as great as the number previously mentioned.

The Rissho Kosei-Kai Modern Translation says:

Good children, the life span I gained through my long practice of the bodhisattva way is still unending and will continue for twice the duration that I described before.

Burton Watson, in his translation for Soka Gakkai in 1993, offers:

Good men, originally I practiced the bodhisattva way, and the life span that I acquired then has yet to come to an end but will last twice the number of years that have already passed.

Tamura declares that the eternal life of the Buddha is shown through infinite, never-ending bodhisattva practice. That seems too narrow to me. The entire Lotus Sutra – Myōhōrengekyō – is realized by endlessly doing bodhisattva practice in this actual world. This is how Namu Myōhōrengekyō is put into practice daily.

As for the Buddha:

I am always thinking:
“How shall I cause all living beings
To enter into the unsurpassed Way
And quickly become Buddhas?”

The eternal life of the Buddha is realized in this vow.