I disagree, and posted both the following quote (as well as its preceding two [or so] paragraphs, not shown here) as the explanation to what 2 Peter 3 is referring to in this verse:
[quoting Gaebelein; but see also the two paragraphs at link that precede this part, for further explanation]
"What, then, does the passage mean? It is very simple after all. He preached by the Spirit, or in the Spirit, that is, the same Spirit who raised Him from among the dead, the Holy Spirit of life and power, to the spirits who are now in prison.
But when the preaching occurred they were not in prison. And who were they? All the wicked dead for 4,000 years? The text makes it clear that they are a special class of people. They were living in the days of Noah. It is incomprehensible how some of these teachers, misinterpreting this passage, can teach that it includes all the lost, or angels which fell, or the righteous dead.
The Spirit of God preached to them, that is, the Spirit who quickened the body of Christ, the same Spirit preached to the generation of unbelievers in the days of Noah. The time of the preaching, then, did not occur between the death and resurrection of Christ, but it took place in Noah’s day.
Christ was not personally, or corporeally present, just as He is not present in person in this age when the gospel is preached; His Spirit is here.
"
So was He present by His Spirit in the days of Noah. It is written: “My Spirit shall not always strive with man, for that he also is flesh; yet his days shall be one hundred and twenty years” (
Genesis 6:3). His Spirit was then on the earth. In long-suffering God was waiting for one hundred and twenty years while the ark was preparing.
His Spirit preached then. But He needed an instrument. The instrument was Noah; in him was the Spirit of Christ and as the preacher of righteousness (2Peter 2:5) he delivered the warning message of an impending judgment to those about him, who did not heed the message, passed on in disobedience, were swept away by the deluge and are now the spirits in prison. As
the Spirit of Christ was in the prophets (
1Peter 1:11) testifying beforehand of the suffering of Christ and the glory that should follow,
so the Spirit of Christ preached through Noah. This is the meaning of this passage, and any other is faulty and unscriptural."
--Gaebelein, Commentary on 2 Peter 3,
https://biblehub.com/commentaries/gaebelein/1_peter/3.htm
[end quoting; bold and underline mine]