Lot's of people repot white pines towards the end of August here. Most of my books recommend it for pines that have been a while between repots or have slowed down in vigor over the growing season.
I have a medium sized white pine that wasn't draining the year I got it. I repotted in in August and it came back with a vengence the following spring.
If you've been watching Q's videos, you've probably noticed that the "rules" that are often considered quite firm in the west, are more like loose guidlines for Japanese growers. Much of that comes from experience I reckon, but the climate also plays a role.
Another example is repotting of deciduous trees. I often see people here say, "when the buds begin to swell", but most people here start repotting their maples in January, and will often continue to do so right into April when they've leafed out without much worry. That being said, much of Japan has much more mild winters than many of you in North America do, so we can get away with more.