Anecdotally, this is true in many places but not true in San Francisco. August tends to be slow for purchases in SF for historic reasons that I don't fully grok, but it shows up in the data and is "common knowledge" among local agents.
Summer is the slow season for real estate in SF. The peaks are in Spring and Fall.
Maybe for SF. But for outlying areas, summer is still peak because parents don't want to move when kids are in school.