Is there any reasons to think that an external power supplies will or can be more quiet?
While perhaps not any quieter than an internal PSU, having an external PSU removes a major source of heat from the case while freeing up a lot of space, making the rest of the system much easier to cool.
Easier to cool means less airflow necessary. Less airflow necessary means fewer, slower fans. Fewer, slower fans means less noise.
External PSUs are surrounded by ambient temperature air and have fewer size restrictions than internal PSUs. Thus, they can be made quieter, although I don't know if the Soldam unit in its stock configuration is particularly quiet.
All of this may not make a difference to most users with mid-tower or bigger cases. It's easy enough to make a very quiet PSU for that size of PC. But one of the major problems with SFF PCs has been that they have these small PSUs that require small, high-speed fans which make an awful lot of noise. Additionally, all the components of an SFF PC (the CPU, the hard drives, etc.) are typically hard to cool because they are so restricted in space. Removing the PSU frees up a lot of space, making it easier to install cooling solutions for the remaining components. For example, my Magic 8 Ball tells me that if Shuttle sold a quiet (thermo controlled 80mm fan, say) PSU for it's "XPC" line, they would sell like mad.
While it's certainly possible to "fake" an external PSU by using extension wires and the like (something I did myself for an SFF PC I once built), Soldam's product (like any commercial solution) is much more polished.