It doesn't surprise me in the slightest.
I'm a supply teacher in primary schools, and have been teaching for nearly 20 years. I've definitely noticed a decrease in fitness levels in kids over that time.
But the quality of teaching has also decreased I'd say.
A lot of schools now out-source their PE teaching to external companies, and it's completely luck of the draw who you get. But most seem to be young-ish men in their early to mid 20s. Some are really good, really inclusive, try to get everyone involved. But a lot are really poor, and many don't care a jot about those "not very sporty" kids one bit.
Yesterday, I supervised in a Year 5 diamond cricket game with one of these coaches. He started by putting 2 sporty boys at the front to pick the teams one at a time - a horribly outdated practice.
Then the sporty boys raced to get to the 4 cricket bats at the same time, leaving all the girls, and less keen boys to have to simply queue up and wait for one of them to get struck out. None of them did. 20 minutes in and it was the 4 original boys still batting. I had to step in and get them off the pitch to let other kids have a turn. They all marched off full of indignation about how dare I? They weren't out! etc etc. Clearly they are used to dominating.
They switched bowlers every 6 balls... 5 boys in a row took a turn before I had to call out, "Next bowler needs to be a girl!" At which point this "coach" pipes up, "Oh yes, next one is a girl everyone!"
And 3 other kids just opted out of the entire game completely! I attempted to cajole them into it, but no success. They spent the entire hour just sitting out on the grass chatting. The coach didn't speak to them a single time.
It was really sad to see this "lesson" to be honest, and I left the field feeling quite angry. I'm considering emailing the Head some feedback.