Answer: ConTeXt includes system fonts by default, unless the user prevents it by two steps. (tested under windows and found similar code for MacOS, but maybe not for linux)
The environment variable
OSFONTDIR must be set and must not be empty. It can point to the desired font directory of fonts you want to use inside ConTeXt or to an empty directory.
At least on windows-systems, the file
\fonts\conf\fonts.conf must also be changed. An XML-entry, by default
<dir>c:/windows/fonts</dir>, must also be changed and set to the desired location of the fonts.
Background: If the environment variable
OSFONTDIR is not set or is set to an empty string, it is filled with the path to the system font directory by
if osfontdir and osfontdir~="" then
elseif osname=="windows" then
elseif osname=="macosx" then