I agree 100% with the spirit of your point and I think imagining a forced bet scenario can help to clarify things. There are three main concepts we want to interpret within the context of the phrasing of the rule: (1) the intended referrent of 'vehicle'; (2) the intended meaning of 'in' the park; (3) the actual intention of the rule regarding emergency vehicles.

This is the scenario: imagine you're forced to wager a nontrivial sum of money on the following bet. You have to write down how you interpreted (1), (2), and (3). Then we randomly pick a real park that has this exact rule phrased in this exact way (I'm hopeful there'll be at least one out there), find the person who wrote the rule, give them your written interpretation, and ask if they agree. You lose if they don't. Notice we're not asking them to also write down a longer interpretation and comparing word-for-word. Just whether they think you got the gist of it.

I would write down that 'vehicle' was intended to refer to motorized passenger vehicles, 'in' was intended to mean that the vehicles shouldn't be in/on water or land within park boundaries, and that the rule wasn't intended to restrict passage to emergency vehicles responding to emergency situations. I expect most people would write something similar if they had real money on the line.

The trouble with the horrible website is it's trying to prove that nebulosity makes content moderation difficult by forcing people to disagree, but this disagreement almost entirely pertains to a point that has nothing to do with nebulosity: the park rule would only ever be written within a wider legal framework and doesn't make sense in isolation.

If I take my answers to (1) and (2), I'm forced to conclude that the emergency vehicles were violating the rule within the ridiculously artificial scenario presented. However, I'm also confident that this rule would only have been written verbatim within a wider legal framework that provided exceptions for emergency vehicles.

Consider self-defence in the context of murder or manslaughter. In the UK at least, the first thing the court does is establish whether the defendant would fit the criteria for murder / manslaughter ignoring the self-defence aspect, because otherwise it's a moot point. Once this is done, they would then establish whether the defence of self-defence also applies, which would then negate the conviction. If you wanted to prove that law is complex because it's hard to define words, would you really make a website that says "Ignore everything else you know and suppose that murder is only defined as killing a person" and then think you're being really smart when people disagree on the scenario involving clear self-defence? Hopefully not, because they're really only disagreeing with being forced to invoke your artificially-restricted definition.

That said, the website demonstrates the real reason why online moderation is hard: because it disproportionately attracts the sorts of people who answered 'yes' to the ISS question in this quiz. So you often end up with lots of users sharing a reasonable consensus on what the rules mean being moderated by a tiny group of... we'll say 'non-representative' moderators. It's a common problem with any banal form of authority, and isn't specific to website moderation at all.