No, it is no hype. The term "informal project management" actually exists.
Informal project management is the management of a temporary endeavor (that may or may not be referred to as "project"), by someone who is not a project manager, without really calling the whole process project management.
Informal project management does not follow a methodology (in contrast with formal project management).
Informal project management is a common way to manage projects in traditional (line) organizations.
I think in a few years from now informal project management will cease to exist when most companies become matrix or projectized.