I'd guess it's as simple as selecting the subject in Photoshop - with a tad more care & attention than I've used below, then leeching out the saturation in the background & tonally balancing towards a sepia effect.

As a very quick demo I did the same thing but made it a pretty garish purple instead.

Once you have your mask you can treat inside & outside of it in totally different ways.

The subjects have been left with realistic colouration, which I think is what is providing the majority of the visual separation - that & the physical separation from the ground, which pushes them into the unsharp area of the background. Note how the effect is less emphatic on the small boy, especially lower, where he's connected to the equally sharp ground at that distance.

I don't think the focus has been played with. I think it was shot on a wide enough aperture that the background is blurred by simple distance. The ground underneath them is still reasonably sharp.
I also don't think it would be compulsory to be using flash, so long as enough light was getting in, or set to a high-enough ISO, to use a short exposure.
The light on the people & the trees seems to match - little to no shadow at all, which matches the OP's description & the almost 'white-out' cloud cover in the back of the shot.

Late addition
I'm not seeing any hint of even a slight fill-flash. Shadows just don't match, & there's not the faintest hint of a catch-light in the eyes.

Additionally, for the 'ooh it's a composite' voices.
The two backgrounds will not overlay, no matter how much you stretch them or play with the perspective - so the 'trick' of extracting the subjects & pasting into a separate shot of the orchard would have required 2 different shots of the background, taken from 2 slightly different places. The photographer would then have had to cut the subjects out of a background they were already in, to replace it with another, taken on the same day at around the same time in approximately the same place... to what end?
That just makes no sense.