Honestly, I've really delved into the specifics of this situation. I was posting from more of an overview of assessing how to apply rules to the general situation, and I think "number of people" is a terrible factor. I also think that a hurricane evacuation is sufficiently foreseeable that people should realize that they need to register early or lose the chance to do so.
It sucks that the legislature didn't expressly account for a possible occurrence like this and I really think the solution here is to work to improve government so things like this can be handled. The legislature should be able to respond quickly to pass an emergency-extension bill, or the law should include provisions for when it can and can't be extended with lines drawn that try to avoid being arbitrary as much as that is possible.
Obviously, while there is no excuse for not preparing for the above in the future, on this occasion they got caught with their pants down. I'm not actually opposed to granting an extension, but at a cursory glance I don't think it should be constitutionally required - unless of course, there is adequate proof that the motives here were political (I am cynical enough to assume they were, but idealistic enough to not permit a judge to rely upon cynicism in making finding of facts). I didn't read the article, but I'll accept your word that he said as much - that he would provide an extension if it helped his party and agree based thereon, that in this situation an extension should be provided.
However, I see nothing wrong if the Legislature passed the law to make it clear that in the future years the deadline will be firm, no if, ands, buts or natural disasters.