Everyone in the ISKCON and Gaudiya Brahman spiritual sect knows about Brahma-samhita and the importance of the Shasta, right from Krishna-Chaitanya, Sril Jiva Gosvami, Bhaktisiddhanta Svami maharaj, so on and so forth. At the end of the fifth chapter (sixty two verses), we find the "panch-sloki vijnana", the five versed knowledge. Krishna (thru Gayatri) is passing on the eternal knowledge to Lord Brahma. The question comes on our minds, which Mahavishnu/vishnu, mentioned in the verse, "atho uvaach mahavishnu....". ?? [The Causal Maha-vishnu (Karnodasayi Vishnu) or the Radical Mool-Vishnu (Garbhodsayi Vishnu) ??]
Well, that is seen from the translation of the verse itself: https://vedabase.io/en/library/bs/5/57/
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 me
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 Me.
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.