Anecdotal information from one person or two is not very valid.
I've been on the summit once per year for forty years, so I have seen a broad spectrum of weather during June and July.
Most of the times I can get by with a couple of thin layers with a good wind shell over that. However, when the wind blows, it can get very cold. I've often had to add a thick sweater and a puffy down jacket, plus a warm hat and gloves. I've been rained on, snowed on, and sleeted on. On top of that, you can get a serious sunburn up there.
Good advice. Even on day hikes I carry cold weather gear. It's a small insurance premium to pay but once or twice every few years, worth it.
