You have to do something like
Text[Style["some text", FontWeight -> weight]]
See this example from the documentation of Text
:
Graphics[{Text[Style["ABCDEF", Large, Bold, Red], {0, 0},
Automatic, {2, 1}], Circle[]}]
which produces

The documentation of Bold
says:
Bold is effectively equivalent to FontWeight->Bold
Edit This might indeed be a bug. Even if the system supports multiple font weights, in Graphics there really only seem two to be available.
weights = {"Thin", "Light", "Plain", "Medium", "SemiBold", "Heavy", "Black", "Fat"};
Row@Table[Style["A", Large, FontWeight -> w], {w, weights}]

Row@Table[ImageCrop@Graphics[#]&@Text@Style["A", Large, FontWeight->w], {w,weights}]
