We cannot say for sure where the try could have been scored but the defender tackled him without the ball. It was maybe fortunate that the attacker had momentum to still touch it down.
Could the attacker, but for the foul play, have gathered and dotted down under the posts possibly but not probably, and for this I use "more likely than not" as the earlier discussion about probability sets the entire range between 0 and 1 but doesn't differentiate.
So although the law allows the PT:
A penalty try is awarded between the goal posts if foul play by the opposing team prevents a probable try from being scored, or scored in a more advantageous position. A player guilty of this must be cautioned and temporarily suspended or sent off. No conversion is attempted.
WB may have considered that a try was a certainty, it seems he thought scoring in a better position was not sufficiently probable to go under the posts. We do expect kickers to pop those conversions over nowadays, you cannot change the decision based upon poor execution by the kicker.
We used to see very few penalty tries and then deliberate knock on, and TMO overlap reviews brought them in greater numbers. Perhaps now offside at try line rucks (long overdue) and tackles from defenders chasing from behind may come into plays that are deemed acceptable to penalise.
Always a judgment and we could have no end of discussion; not only if a try was prevented but could the try have been scored in a better place.
See Bristol v Quins for a differing perspective!