Contrary to what some people will tell you, the President is "not" in fact attempting to overrule the Court decision. The court actually ruled 9-0 that the citizenship question was legal and could be on the census.
What the courts ruled against was the process of how the question was added. They used the Administrative Procedures Act as the legal justification for review. This act provides the courts with some degree of oversight into Federal agencies, generally to determine if the agency, people, or person making the decision has the authority to do so, if the action itself is constitutional, and if the reasoning is not arbitrary or capricious.
So the legal question is not whether or not the citizenship question can be added (everyone agrees it can be). The legal question is whether or not it was added "legally". Since the APA governs government "agencies" and does not likely apply to the President, the courts do not have the same authority to block him (at least not under the same reasoning).
This doesn't mean that they cannot come up with another reason to block the President. But keeping in mind (and this cannot be stressed enough) that because the question itself is not what the court has ruled against, the addition of the question by another means is not the same thing as overruling the decision.
Unless of course, the court was using the APA as an "excuse" overrule a perfectly valid question, because they didn't like the question itself (but knew there was no legal means to prevent it from being on the census). To some degree, Trump is calling the bluff.
In fact, I would go so far as to say that if Trump's executive action ultimately prevails, and the question is on the census, that the courts should still resolve the original issue of whether or not the agency in question provided a reasonable reason. Leaving that part of it out there at this point would be troublesome and leave a lot of unanswered questions... all thanks to a wishy washy Chief Justice who is willing to do dumb things for whatever reasons he has.
UPDATE: One thing I failed to consider is the executive action actually now provides the Agency with a reasonable reason to add the question. They are following an executive order. Does the court have the power to tell an agency that they cannot follow an executive order (especially on an issue that they suggest is perfectly legal). Perhaps the execute order is not only unreviewable under the APA, but it also provides the agency in question with the needed reasoning.
