Matt 3:3 οὗτος γάρ ἐστιν ὁ ῥηθεὶς διὰ Ἠσαΐου τοῦ προφήτου

[Jonathan Robie]

οὗτος γάρ ἐστιν ὁ ῥηθεὶς διὰ Ἠσαΐου τοῦ προφήτου λέγοντος, Φωνὴ βοῶντος ἐν τῇ ἐρήμῳ· Ἑτοιμάσατε τὴν ὁδὸν κυρίου, εὐθείας ποιεῖτε τὰς τρίβους αὐτοῦ.

How does the historical present ἐστιν function in this verse?

[Daniel Semler]

οὗτος γάρ ἐστιν ὁ ῥηθεὶς διὰ Ἠσαΐου τοῦ προφήτου λέγοντος, Φωνὴ βοῶντος ἐν τῇ ἐρήμῳ· Ἑτοιμάσατε τὴν ὁδὸν κυρίου, εὐθείας ποιεῖτε τὰς τρίβους αὐτοῦ.

How does the historical present ἐστιν function in this verse?

My first reaction was, is it an historical present ?

Isn't this just "this is what was said ....", thus copula ? That said, I haven't tried to expand the context.
I'll note, Wallace claims there are no HP's in εἰμί in the NT or other Greek.

Significantly, the one verb that is not used as a historical present is the equative verb (εἰμί).43 Also, when γίνομαι functions as an equative verb, it is not used as a historical present. However, it may function as other than an equative verb at times.44

Daniel B. Wallace, Greek Grammar Beyond the Basics: an Exegetical Syntax of the New Testament, Accordance electronic ed. (Grand Rapids: Zondervan, 1996), 529.
https://accordance.bible/link/read/Wallace_Greek#8951

And the notes

43 Although the copula may be used as a present tense retained in indirect discourse, which is an entirely different idiom. Much confusion has arisen over the similarities in translation between these two.

44 Hawkins lists two or three instances with γίνομαι; in neither instance is it functioning as an equative verb (cf. Mark 2:15).

Thx
D

[Stephen Carlson]

οὗτος γάρ ἐστιν ὁ ῥηθεὶς διὰ Ἠσαΐου τοῦ προφήτου λέγοντος, Φωνὴ βοῶντος ἐν τῇ ἐρήμῳ· Ἑτοιμάσατε τὴν ὁδὸν κυρίου, εὐθείας ποιεῖτε τὰς τρίβους αὐτοῦ.

How does the historical present ἐστιν function in this verse?

It’s not. This is an aside from the narrative.