Yeah its not going to be taken seriously since Mueller's mandate is very broad and specifically stipulates that he can investigate "...... any links and/or coordination bet ween the Russian government and individuals associated with the campaign of President Donald Trump; and any matters that arose or may arise directly from the investigation....".
https://www.justice.gov/opa/press-release/file/967231/download
So basically he can go after anything that relates to Trump or Russia and/or anything that arises as a result of his initial inquiry. That's very broad and can obviously include Trump's taxes, past real estate deals etc, so Trump's lawyers don't have a leg to stand on.