This is the cited article. Thankfully, the interviewee provided a scan of the police letter he received, so the rest of this question is relatively easy to answer.
He was specifically charged with Störung der öffentlichen Ordnung (lit. Disturbance of the public order) persuant to § 81(1) of the Sicherheitspolizeigesetz, which reads (after putting it through Google Translate):
Who by a behavior that is likely to arouse legitimate annoyance, disturbs public order, commits an administrative offense and is punishable by a fine of up to 500 euros, unless the behavior is justified, in particular by the use of a constitutionally guaranteed right , In the event of aggravating circumstances, instead of a fine, imprisonment can be imprisoned for up to one week, or up to two weeks for repeated offenses.
This law is almost certainly constitutional as it specifically allows exercise of constitutional rights. Note this also includes human rights, as Austria has included the European Convention on Human Rights as part of its constitution.
Given that, I'm guessing if the interviewee had wanted to, he would have had a decent shot at having this charge dismissed.