Probably the guy who produced that part of the graphic was not told what a thermonuclear warhead actually looks like, because he didn't need to know, so he just whipped up his own idea of it from speculative public images. Knowing that the graphic came from somebody who didn't actually know anything, the censors didn't see the need to worry about it.
That is not how nuclear secrets work. The US Department of Energy holds that restricted data (a special kind of classification that only applies to nuclear secrets) is "born secret". That means, even if you come up with a concept for a nuclear weapon completely independently without ever talking to anyone, it is considered classified information that you are not allowed to redistribute. This doctrine is highly controversial and the one time it has been tried in court the verdict was inconclusive, but to this day it is how the DoE interprets the Atomic Energy Act of 1954.
In general this is very precarious to attempt to enforce, of course. If the DoE sues someone because they published their nuclear weapon designs, that'd be seen as a tacit admission that the design could potentially work. Nevertheless they actually did do this at one point (United States v. Progressive, Inc., 1979).