You are correct. However, if you mulch it well and keep it watered, it will do fine. Plants you already have on hand should not wait until autumn to go in, with soil around them they will be less prone to drying out than if still in pots - esp. if you wash the potting soil off and open the roots up when planting.

Some trees start to make annual autumn burst of root elongation (60% occurs in fall) rather early in the season, those planted in summer will be in position for this well in advance. Root elongation is prompted by formation of terminal, overwintering buds at branch tips, any tree that has set these may be rooting out already. I have seen vine maples in sawdust beds that had their fall root elongation well under way in August.