It is "doubly so" because the border guard was wrong to judge the content as "lese majeste" on account of JD himself sharing it, and was wrong that "lese majest

It is "doubly so" because the border guard was wrong to judge the content as "lese majeste" on account of JD himself sharing it, and was wrong that "lese majeste" is applicable in America. The guard was wrong, and even if one doesn't agree with one of the reasons they were wrong because they don't share those values, the guard would still be wrong for the other reason. Therefore they were doubly wrong.