ARC has two test sets that are not posted on the Internet. One is kept completely private and never shared. It is used when testing open source models and the models are run locally with no internet access. The other test set is used when testing closed source models that are only available as APIs. So it could be leaked in theory, but it is still not posted on the internet and can't be in any web crawls.

You could argue that the models can get an advantage by looking at the training set which is on the internet. But all of the tasks are unique and generalizing from the training set to the test set is the whole point of the benchmark. So it's not a serious objection.