Bhashya
यदि च पुण्यलोकातिशयमात्रममृतत्वमभविष्यत्, ततःपुण्यलोकत्वाद्विभक्तं नावक्ष्यत् ।विभक्तोपदेशाच्च आत्यन्ति-कममृतत्वमिति गम्यते ।अत्र च आश्रमधर्मफलोपन्यासप्रणवसेवास्तुत्यर्थं, न तत्फलविध्यर्थं, स्तुतये च प्रणवसेवाया, आश्रमधर्मफलविधये च, इति हि भिद्येत वाक्यम् ।तस्मात्स्मृतिप्रसिद्धाश्रमफलानुवादेन प्रणवसेवाफलममृतत्वं ब्रुवन् प्रणवसेवा स्तौति ।यथा पूणवर्मण सेवा भक्तपरिधानमात्रफला, राजवर्मणस्तु सेवा राज्यतुल्यफलेति- तद्वत् ।प्रणवश्च तत्सत्यं परं ब्रह्म तत्प्रतीकत्वात् ।'एतद्ध्येवाक्षरं ब्रह्म एतद्ध्येवाक्षरं परम्' इत्याद्याम्नानात्काठके, युक्तं तत्सेवातोऽमृतत्वम् ॥
In plain words
Had immortality been nothing more than an elevated condition of the meritorious worlds, the text would not have distinguished it from them. This distinct teaching proves that immortality is absolute and unconditioned. Furthermore, mentioning the rewards of the various life stages here is meant to glorify the meditation on OM, not to prescribe those worldly rewards. Just as serving a minor lord yields mere garments while serving a great king yields an entire kingdom, so it is here...
Close to the Sanskrit
And if immortality had been merely an excess of meritorious worlds, then the scripture would not have spoken of it as separate from the state of meritorious worlds. From the separate teaching, it is understood that immortality is absolute. Here, the mention of the fruits of ashrama duties is for the purpose of praising the worship of Pranava, not for enjoining those fruits...