I think this is exactly what has happened.
It isn't a massive leap for me at all to believe he might well have found numerous aspects of his childhood difficult. I mean, most people do, so why not him? I guess the difference when those things stem from a variety of privilege is a) nobody gives a sh"t and thinks you just should stop whining and b) it is probably easier to "blame" parents than if they were struggling to provide financially etc. Yet they no doubt had plenty of other pressures of a different nature. For eg, the Loos thing must have been hard for VB. Yes, affairs are common but most people don't weather "did he/didn't he" fascination in the media.
FWIW I would have loathed having B's childhood.
So yes, the complaints might have been there for a long time, but I think the relationship between VB and N has probably set the match to the bonfire of issues he carries from his childhood (as do so many people; it's not unusual).
The IG statement, I think, suggests this. There are not a whole lot of references to things I could really feel sorry about, such as paps when he was a little boy, the Loos saga etc. Instead, the focus was essentially on the N/V clashes and things like V trying to "steal" the first dance. To me that's something that grates on the bride's nerves - and when that bride has presumably had a lot of getting her own way, that is probably even more inflammatory. Add to that that the Beckhams no doubt struggle in the face of a wealth that dwarfs theirs - something they have underpinned their "brand" with - it was never going to be an easy MIL/DIL relationship.
I think there is a lot of dysfunctionality in it all, but probably no single player is "the one" at fault. It's the same old, same old of complicated dynamics where everybody's insecurities and resentments and missteps add to the mix - then it all gets writ large because they have a public audience.
He hasn't played it perfectly, but then I suspect none of them have, and I am not unable to feel sorry for him in some respects.