Referee right?

Referee wrong.

Naas Olivier of the Griquas kicks a high ball. Herkie Kruger of the Golden Lions catches the ball and is immediately flattened by Bjorn Basson of Griquas.

The referee decides that the situation is unplayable and awards a scrum to the Golden Lions. There is a dissenting voice amongst the Griquas but the referee quietens it and explains that because the ball was caught the scrum went to the Golden Lions.

The dissenting voice was right.

Was there a maul here?

No.

There was just a tackle, and then the ball became unplayable.

The scrum is awarded according to the tackle law.

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.

It has nothing to do with whether the ball was caught or not.

And if it had become 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.

Again it has nothing to do with whether the ball was caught or not.

The change comes with the maul, which this was not. It is only at the maul that the caught ball is significant.

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.
‘Direct from an opponent’s kick’ means the ball did not touch another player or the ground before the player caught it.

The catcher is significant only when a maul ends unsuccessfully and a scrum is to be formed.

This is a common error.

It should have been Griquas ball.