This verse is one of Calvinists most misinterpreted verses and all because you do not look at the context. Look at verse 18. It's saying that many antichrists have come. Christ was a true prophet and a true teacher. Therefore antichrists here refers to false prophets and false teachers, not just your average nonbeliever. They were also "tares in the wheat, planted by the devil", evil leaders who were trying to deceive real disciples, and turn them away from the truth. But they quickly realized that many mature Christians knew their false teaching and evil plans, so they had to pack up and go somewhere else where they would not be detected - somewhere where people would actually welcome them and follow their false teaching.
So those antichrist's mentioned in verse 18 were never really true saints, as you put it.
BUT, we have NO indication that Hymenaeus and Alexander were antichrists. They could very well have been true believers, men who had faith and a good conscience, but then they rejected their faith and good conscience. Paul said that their faith was "shipwrecked". So Paul is indicating that they must have had real faith to begin with - otherwise they would have had nothing to shipwreck.
So here is an example of genuine believers who later decided to reject their faith in Jesus - and they actually lost their salvation - they were shipwrecked. Paul "delivered them over to Satan that they may learn not to blaspheme". If they were repentant and teachable, it's possible that they came back to Jesus. If not, they were lost.