Not true. A back button is meant to "go back to previous state". When you click a link, then back, you will go back to the previous page or position on the same page depending on the link you clicked.

Similarly, if you enable a lightbox and your page goes behind a modal image, hitting back will behave exactly the way it should, i.e. take you back to your previous state.

I'll go one step further and say that a lightbox implementation that doesn't do this should be considered broken since it breaks user expectations. This does raise the question of judging a user's intent when he presses the back/forward button while there is something modal on his screen, but let's save that for some other time.