Penalty try? What for? That is heavy.

The Pumas attack down the left and wing Allistair Kettledas darts ahead. Hemmed in by three Golden Lions players, he passes inside to outside centre Tian Meyer who darts for the Golden Lions line.

Meyer is about five metres from the line with nobody in front of him when Franco van der Merwe, the big Golden Lions lock reaches out, grabs Meyer by the collar and pulls him back to earth. The try is not scored.

The referee consults his assistant in a conversation that is concise, clear and well organised. They agree that it was a high tackle, that it was foul play, that it prevented a probable try and that the sanction should be a penalty try and a yellow card.

Van der Merwe sets off for the sin bin and the referee for the posts to award the penalty try.

OK?

Law 10 deals with foul play.

Law 10.4 (e) Dangerous tackling. A player must not tackle (or try to tackle) an opponent above the line of the shoulders even if the tackle starts below the line of the shoulders. A tackle around the opponent’s neck or head is dangerous play.
Sanction: Penalty kick

Meyer's collar is above the line of the shoulders and the way he was jerked back was certainly dangerous.

Then what about the penalty try and the yellow card? Was it worth a yellow card?

Law 10.2 UNFAIR PLAY
(a) Intentionally Offending. A player must not intentionally infringe any Law of the Game, or play unfairly. The player who intentionally offends must be either admonished, or cautioned that a send off will result if the offence or a similar offence is committed, or sent off.
Sanction: Penalty kick
A penalty try must be awarded if the offence prevents a try that would probably otherwise have been scored. A player who prevents a try being scored through foul play must either be cautioned and temporarily suspended or sent off.

Van der Merwe acted intentionally.
Meyer would most probably have scored if Van der Merwe had not tackled him illegally and dangerously.
Van der Merwe prevented a try from being scored through foul play.
The referee had to award a penalty try.
The referee had to deal with Van der Merwe and the option he had was a yellow card (Temporary suspension) or a red card (sending off).

The referee certainly acted correctly.

What about the argument that Van der Merwe could have prevented the try if he had tackled legally? The argument is spurious. He did not act legally and his illegal action prevented a try. The argument is rather like saying his wife would be alive if he'd not hit her over the head with a meat axe. He did it, it was wrong and he must bear the consequences.