Ultimately, the decision tree for the final decision boils down to something like the below:
1. Had a ruck formed by the time that Tizzano attempted a jackal?
No -- he was the first to arrive after the tackle. They did not arrive simultaneously as claimed by the referee -- he was evidently knocked off the ball by the Lion's player, which suggests he was there first.
This means that he wasn't engaged by any opposing player prior to competing for the ball, and therefore no ruck had formed.
Given that, the law which suggests that his head has to remain above his hips isn't relevant and he is allowed to compete for the ball as long as he supports his own body weight.
2. Did the Lions player strike above the shoulder line?
Yes -- this is abundantly clear, both in real time viewing and even clearer when you slow the footage down (or consider it frame by frame).
The contact was to the back of the neck, and then slides up to head contact as Tizzano is knocked backwards.
By the letter of the law, this is a penalty infringement at the very least.
3. Is this ruling this a penalty consistent with previous interpretations of the law or previous rulings?
Yes -- at the very least, both Bundee Aki and Brodie Retallick have received red cards for the same infringement relatively recently (e.g. within the last 3 years).
Countless other examples can be found.
(speaking to a common Lion's talking point)
4. How is the Lion's player supposed to clear Tizzano out legally?
It is indeed difficult to clean him out legally, but that doesn't mean that you're allowed to use illegal means to clean him out - it might simply mean that you have been outplayed in that moment and that you are unable to win back possession legally.
The Lion's player has to get lower than Tizzano to blast him off the ball, which indeed would've been very difficult in this situation given how well Tizzano was competing for the ball.
This is the key point -- it's the Lion's responsibility to arrive to the ball before Tizzano to secure their possession. If they did, his arrival would constitute a ruck and he would be unable to compete for the ball. In this situation, the Lion's player arrived second and Tizzano was set up sufficiently low to secure possession.
This means that Tizzano had rights to the ball and the Lion's player didn't have the means to legally knock him off the ball.
5. Did Tizzano dive?
Maybe, maybe not.
It seems to me that he put on a bit of a performance to draw attention to the illegal play, but this does not mean that the Lion's cleanout was not illegal in and of itself.
I've never had a 110kg forward throw himself off his feet, smashing the back of my neck with his shoulder. I imagine it hurts, and I imagine that you'd potentially want to make that as clear as possible to the referee, particularly in the context of a game winning moment that will keep a once-every-12-year series alive.
The worst part of all of this is that World Rugby will defend the decision here -- they simply refuse to ever admit that they are wrong. The fact that this is a penalty is black-and-white, and it's impossible to not feel deeply wronged here, particularly in the context of previous seasons (e.g. Foley being penalised for time wasting against the All Blacks).
It would be nice if a call went our way every now and then. God knows we need it.