What they mean by "specific learned skill" is that each task illustrates the use of certain "core knowledge priors" that François Chollet has claimed are necessary to solve said tasks. You can find this claim in Chollet's white paper that introduced ARC, linked below:

On the Measure of Intelligence

https://arxiv.org/abs/1911.01547

"Core knowledge priors" are a concept from psychology and cognitive science as far as I can tell.

To be clear, other than Chollet's claim that the "core knowledge priors" are necessary to solve ARC tasks, as far as I can tell, there is no other reason to assume so and every single system that has posted any above-0% results so far does not make any attempt to use that concept, so at the very least we can know that the tasks solved so far do not need any core knowledge priors to be solved.

But, just to be perfectly clear: when results are posted, they are measured by simple comparison of the target output grids with the output grids generated by a system. Not by comparing the method used to solve a task.

Also, if I may be critical: you can find this information all over the place online. It takes a bit of reading I suppose, but it's public information.