That is true, although really disabling the context menu is just a hindrance, rather than an absolute solution. They can be bypassed, although the implementation can change the level of difficulty. The most common way is a using JavaScript - all you need to do is turn off JavaScript execution or using something like ScriptBlocker. Ways involving CSS (the styles applied to make the page look nice) just require more work and some familiarity with web design, typically.
However, in the case we're discussing, I don't think this is the issue. Ken still has the context menu, as he can select "Save image as"; however, the "Copy Image" option is missing. My guess is that this will affect all websites he visits, not just Google.