Look, this just isn't true. I've heard it repeated a lot online, but I've never encountered it myself.
For context, I have a UK PhD, not from Oxford or Cambridge, and I got hired as a postdoc in a top-10 school in the US doing really exciting research.
There are differences between PhDs in US and the UK and Europe, but the differences are complex... there's no clear answer that one is better over the other.
It's a little unfortunate, because I've known Americans who were put off a UK-based PhD because they think professors look down on them, but I've seen little evidence to back it up. My colleagues come from all over the world, and they're all super smart, and everyone's treated equally and with respect. When it comes down to it, the quality of your research, and your ability to work hard, is far more important than what country or institution you're from.
(There are also a lot of anecdotes out there... in this thread, someone who completed early (congrats btw!), someone else with 20+ papers... you can be guaranteed there are people reading this thread who might be discouraged reading this. The one unifying factor with PhDs is that personal experience is highly variable, and artificial targets (finish in x years with y papers etc.) do not correlate with later success or the value of your research. So for anyone still slogging through who's reading this, good luck!)