First off, there is no such thing as camera being more or less silent than another camera, mostly because if it's silent -- it makes no noise. So it's not possible to make less noise if there is none or make more noise if there is none. It's like being more dead or less dead. Dead is dead.
But I understand the intent of the message.
It's correct that leaf shutters generally will emit less noise than a focal plane shutter. However, some cameras -- such as the Voigtlander Prominent -- aren't that subtle. Others, such as the Voigtlander Vito I, II and IIa are reasonably quiet.
The Retina IIIS has the quietest leaf shutter that I've encountered. The Yashica Electros are sort of quiet too, until you advance the film (clunk).
My Contax IIa (black dial) set at 1/25 is a very quiet camera. I've used it inside a church at that speed, and no one turned their head. The loudest camera that I have by a wide margin is the Rolleiflex SL66.
In the mid-1970s, Olympus (again) led the way, downsizing SLRs and making them quieter. The OM-1 is a reasonably quiet camera, as are some of its contemporaries: Pentax MX (only partly successful in damping the shutter sound), Canon AE-1 and the Minolta XD-11.