// Brexiteers must be wondering how our former Prime Minister who still has an interest in politics and world affairs (and an interest in the wellbeing of the UK as a whole) can have such a diametrically opposed view to them.//
No they're not wondering that at all.
The reason Mr Major has diametrically opposed views is that he is an ardent Europhile. In 1992 (which, uncoincidentally, is the date I decided I would vote to leave the EU if ever given the chance) he paved the way for the Brexit referendum by signing the Maastricht Treaty on behalf of the UK. This Treaty established the European Union and set it onto the path towards a federal state, which it still ruthlessly pursues today.
There was widespread opposition to the Treaty among both the Conservative and Labour parties (though for different reasons) and Major called a vote of confidence in his administration. This was so that Tory MPs (some of whom planned to vote against the European Communities Amendment Act 1993 which was to allow ratification of the Treaty) could be whipped into line. The confidence vote was won and the Treaty subsequently ratified.
This was a huge change to the UK's Constitutional position and it should have been put to a referendum rather than a whipped vote in the Commons (and if it had it was very likely to have succeeded). But it wasn't and then began the process of the EU's "ever closer union" to which most people who voted to leave in 2016 objected.
I have to thank Mr Major for signing the Maastricht Treaty. That one action alone probably did more than anything else to set the path towards the UK's recent departure from the EU. Had he called a referendum in 1992 and, as I suspect it would, the result was to remain, it would have been far more difficult to make a case for leaving. It's just a pity that it took a quarter of a century.
So before you pose "puzzles" which you seemingly know little about, you need to study what happened and why. There is no puzzle at all why Mr Major now describes Brexit as a mistake. He invested his entire political future in pushing Maastricht through Parliament and thought he'd cracked it. Alas he only postponed the inevitable and revenge, as they say, is a dish best served cold.