The proper interpretation is that the Lord did indeed preach to those sinners who are now dead, but He did so when they were alive and had the ability to hear with their ears and repent. And the Lord accomplished that through His prophet, Noah, a "preacher of righteousness" , but we all know how many listened.
“18 For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit: 19 By which also (the Holy Spirit) he went and preached unto the spirits in prison; (prison of sin) 20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. ”
1 Peter 3:18-20 KJV
Think carefully about what it actually says. The fire shall never be quenched. Right? In other words, no-one it anything, can put it out. What it doesn't say is that it won't go out of its own accord once the fuel is spent. Just like a normal fire.
Smoke (and ashes...see Malachi) is always the last vestige of any fire long after the fire itself has gone out. There is nothing in those texts to suggest the torment never stops, or those suffering never die at the end of it.
Note...“And these shall go away into everlasting punishment: but the righteous into life eternal. ”
Matthew 25:46 KJV
...that it says eternal punishment. That punishment, echoed throughout Scripture, is death. Not punishing as continuing action.