Well, that is seen from the translation of the verse itself:
Śrī brahma-saṁhitā 5.57
athovāca mahā-viṣṇur bhagavantaṁ prajāpatim brahman mahattva-vijñāne
prajā-sarge ca cen matiḥ pañca-ślokīm imāṁ vidyāṁ vatsa dattāṁ nibodha
atha — then; uvāca — said; mahā-viṣṇuḥ — the Supreme Lord; bhagavantam
— unto the glorious; prajāpatim — Lord Brahmā; brahman — O Brahmā;
mahattva — of the glory (of Godhead); vijñāne — in real knowledge;
prajā-sarge — in creating offspring; ca — and; cet — if; matiḥ — the
inclination; pañca-ślokīm — five ślokas; imām — this; vidyām —
science; vatsa — O beloved; dattām — given; nibodha — hear; me — from
On hearing these hymns containing the essence of the
truth, the Supreme Lord Kṛṣṇa said to Brahmā, "Brahmā, if you
experience the inclination to create offspring by being endowed with
the real knowledge of the glory of Godhead, listen, My beloved, from
Me to this science set forth in the following five ślokas.
So it was Lord Krishna who is called as mahā-viṣṇur in the verse, its him who spoke those verses to Brahma. Nothing unusual about that. Lord Krishna can be called as Maha Vishnu, or just as Vishnu because He really is none other than Lord Vishnu himself. In Hindu scriptures we can find many examples where Lord Krishna is called with various names Vishnu, Vasudeva, Keshava, etc.