Well, I don't the context will allow us to say that it's referring to their whole experience, including their baptism. Everything Peter says in
Acts 15 about the Gentiles’ hearts is explicitly connected with God pouring out the Spirit on them. Peter said that he preached the gospel to them and they believed. How did he know they believed? Because, when he preached the gospel to them, God knew their hearts—he knew they had come to faith—and at once poured out his Spirit on them in response to their faith. God knew their hearts and so cleansed their hearts. All this explicitly happened before they were baptized.
It’s important to note that Peter never mentions baptizing them in his argument. And there's a reason for that. Just like in
Acts 11, when he had to defend himself against some Jewish Christians who were attacking him for going to the Gentiles in the first place, Peter focuses exclusively on what God had done. It wouldn't have strengthened his argument against his opponents to focus on anything he had done—like baptizing Cornelius—so Peter mentions only God’s deeds to refute his opponents. God’s actions had brought about the cleansing of their hearts; it had nothing to do with Peter baptizing them. In cleansing them and making no distinction between them and Jewish believers when he poured his Spirit out upon them, God proved he had accepted them as they were without them having to be circumcised.
Right, and this shows they were cleansed of sin before their baptisms. Peter’s promise was that whoever comes to faith in Christ will have their sins forgiven and removed. Their receiving the Spirit showed they had come to faith in Christ and had received this forgiveness. God knew their heart and so cleansed their heart of sin as he poured out the Spirit on them.
When God poured out the Spirit on them, we know from other passages of Scripture that at that moment they were sanctified by the Spirit and born of the Spirit and raised from the dead by the Spirit. A person cannot experience being made holy and being given new life by the Spirit while what had made them unholy and in need of new life in the first place, their sin, still remains. To come to life, what had caused death had to be removed. To be made holy, what had defiled had to be cleansed.