We’re the cryptographers who wrote it allowed to leave the country?

They were NSA employees and IBM engineers. There are no blanket prohibitions on travel for NSA folks, although I believe there are more or less 'don't go there' advisories about specific places. I also imagine there are folks there who know things that put them in special travel categories.

I don't think the IBMers had any special conditions imposed on them.

I'd bet they are closely tracked from a certain distance and, whenever they buy a ticket or cross a border, someone gets an alert.