it depends, may be there were about to witness something that police did not want them to see... so detain and claim an error later...
It may be, but I find it unlikely.
What we know is that two guys were detained for some time and later released.
How long there were detained? We don't know for sure but probably not days or weeks (otherwise they would have told us).
Were they the only detained people after the rock throw? We don't know for sure but, again, probably not because otherwise they would have told us.
Were their cameras taken away and/or their photos confiscated or destroyed?
Again, probably not.
That's why I can't see an intentional journalist detainment: there are none of the sign the detainment was "journalistic motivated"