'Not from a kick-off.'

Which is irrelevant.

Brock James of Clermont kicks off after Ulster score. Stefan Terblanche of  Ulster jumps to catch the ball. Julien Bonnaire of Clermont tackles him. They go to ground immediately. It is a tackle. It is not a maul. It is a tackle.

A heap of players occurs and the referee decides that the ball is unplayable. He awards a scrum to Clermont. Rory Best of Ulster asks gently if it should not be an Ulster put-in as it was from a kick.

The referee says: 'Not from a kick-off.'

In fact that it was from a kick at all is irrelevant because there was no maul. It is only from a maul when a kick is caught that there is any possibility of a scrum to the catcher's team.

The referee was right when he said that even that did not apply from a kick-off, thereby perpetuating the erroneous belief that it is always the catcher's ball from a kick, which is just not the case.

This was a tackle.

The award of a scrum when the ball is unplayable at a tackle applies.

Law 15.8 DOUBT ABOUT FAILURE TO COMPLY
If the ball becomes unplayable at a tackle and there is doubt about which player did not conform to Law, the referee orders a scrum immediately with the throw-in by the team that was moving forward prior to the stoppage or, if no team was moving forward, by the attacking team.

That is the same criterion at a ruck.,

Law 16.7 UNSUCCESSFUL END TO A RUCK
(a) A ruck ends unsuccessfully when the ball becomes unplayable and a scrum is ordered. The team that was moving forward immediately before the ball became unplayable in the ruck throws in the ball.
If neither team was moving forward, or if the referee cannot decide which team was moving forward before the ball became unplayable in the ruck, the team that was moving forward before the ruck began throws in the ball.
If neither team was moving forward, then the attacking team throws in the ball.

In both cases it is about the team moving forward. In this case the referee decides that Clermont were the team moving forward. They were also the attacking team.

Let's look at this catcher thing and hope that players and referees become clear about it.

Law 17.6 (h) Scrum after a maul when catcher is held. If a player catches the ball direct from an opponent’s kick, except from a kick-off or a drop-out, and the player is immediately held by an opponent, a maul may form. Then if the maul remains stationary, stops moving forward for longer than 5 seconds, or if the ball becomes unplayable, and a scrum is ordered, the team of the ball-catcher throws in the ball.

This applies only to a maul. There was no maul in this case. This does not apply. All that business about catching and a kick-off is irrelevant.