Sometimes with mild winters or with a very long growing season you can get flowering on "new" growth. I would not consider it a given however and it may not repeat itself often.

In my climate, hydrangeas tend to have a very long bloom season and it is often possible to get flowers from this season's growth. Especially with oakleaf, where the flowers are produced at the terminal end of the branches, it is very obvious that some of the flowers are generating from new growth.

This spring and summer, I had so many more hydrangeas bloom than ever before. I credit it to last summer being longer and hotter by far than normal. Call me crazy, :-) but it's been unusual to say the least.

My climbing hydrangea had 20+ blooms, and it normally has just a couple. Multiple oakleafs bloomed (and one bloomed on new wood like yours), and I'd had them for 6 - 8 years with zero bloom. I think part of their normal non-blooming issue is too much shade.

I had a couple Endless Summers bloom a lot this year. They're in a fair amount of shade and didn't even bloom last year when I protected them and their branches survived.

I also have a Pink Diamond in too much shade that rarely got much bloom and it's loaded this year. (And yes, I should move all of these to more sun...if I had it, and yes, I have a lot of Annabelles and Incrediballs and they bloom fine.)

Our winter wasn't mild or unusual in any way.

I have some Endless Summers in an eastern exposure, which I cover and bloomed on old wood both last year and this year. But last year, with the warmth and sun, they just kept right on blooming all summer. This year the new growth has no buds.

FWIW, it just seems that the heat and maybe even the stress of last summer encouraged a lot more bloom THIS year on hydrangeas.

Enjoy all those blooms! August is the hottest month here so hardly anything wants to bloom now. Not even the roses.