The lawsuit says publishing a voted ballot on social media can be a powerful form of political expression.
Sorry. Campaigning by the voting booths or threatening to hurt people who don't vote or who do vote differently from you would also be some powerful forms of political expression, but all those are also prohibited by lawful place and manner restrictions on free speech.
There are certain places where no public expression is allowed, and the voting booth is one of them, unless your 'selfie' is to expose some newsworthy thing, and not, say, what your votes were....
In other words.... campaigning, or taking selfies is prohibited, regardless of the content of your message or who you voted for, so it's not a particular restriction based on content of your message, so it's not considered an infringement on free speech rights.