The Thess material was about the 1st century situation because Paul was quite sure the 2nd coming was soon to take place, immediately after the mess in Jerusalem. He (alone) does not seem to allow for any delay like the others. It's one of the things that Peter says is 'hard to understand' about his letters.
Nonsense - Peter is clearly asserting that those Israelites for whom Paul's assertions about God's delaying of His pending wrath in His longsuffering prior to His fulfilling of His Promise unto Israel is hard for said Israelites reading of it to understand, thus their twisting both Peter's and Paul's writings about said delay, as their Israelite forefathers before them did with the writings of Israel's Prophets concerning God's prior, delayed Promises unto Israel.
He was referring to the gainsaying Isralites he began the chapter with.
2 Peter 3:1 This second epistle, beloved, I now write unto you; in both which I stir up your pure minds by way of remembrance: 3:2 That ye may be mindful of the words which were spoken before by the holy prophets, and of the commandment of us the apostles of the Lord and Saviour: 3:3 Knowing this first, that there shall come in the last days scoffers, walking after their own lusts, 3:4 And saying, Where is the promise of his coming? for since the fathers fell asleep, all things continue as they were from the beginning of the creation. 3:5 For this they willingly are ignorant of, that by the word of God the heavens were of old, and the earth standing out of the water and in the water: 3:6 Whereby the world that then was, being overflowed with water, perished: 3:7 But the heavens and the earth, which are now, by the same word are kept in store, reserved unto fire against the day of judgment and perdition of ungodly men. 3:8 But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day. 3:9 The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance. 3:10 But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up. 3:11 Seeing then that all these things shall be dissolved, what manner of persons ought ye to be in all holy conversation and godliness, 3:12 Looking for and hasting unto the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat? 3:13 Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness. 3:14 Wherefore, beloved, seeing that ye look for such things, be diligent that ye may be found of him in peace, without spot, and blameless. 3:15 And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; 3:16 As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction. 3:17 Ye therefore, beloved, seeing ye know these things before, beware lest ye also, being led away with the error of the wicked, fall from your own stedfastness. 3:18 But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ. To him be glory both now and for ever. Amen.
