There is a lot of truth in this.
I went to a top 3 private school in the UK.
Undergrad at Oxbridge equivalent in UK (v niche).
Masters at Oxbridge equivalent abroad.
Now live in a 3rd country, educating my DC at Westminster-equivalent school.
I can't express how mediocre most British private school kids' education is compared to the rigour I see here. It's shocking to me, as it wasn't always thus. So much dumbing down. (I have relatives and friends' children to look at.)
Add to that the weird obsession in the UK with state versus private, Oxbridge versus RG versus other. Parents lose their minds over the most minor things, not seeing the wood for the trees, while also resting on decades-old laurels.
The rest of the world has moved on in leaps and bounds. It's like they're all driving Maclaren supercars (at the top levels) while the UK is chugging along in its good old vintage Jag. Of course, there are absolutely stellar homegrown British students, but nowhere near as many as there were 30-odd years ago and DEFINITELY not compared to worldwide competition.