I don’t think they can really. I don’t work in that stuff, but skipping isn’t included in YT analytics from what I’ve read. I would bet they rely on something like average view percentage to just make assumptions. For example, if a content creator places the sponsor bit in the first 10% of the video, and average view percentage for that video is 80%, then it is assumed the sponsor bit was watched. I wouldn’t be surprised if sponsors require some form of transparency in analytic reporting for content creators to get paid.
I also would figure that YouTube, as it has no bearing on their revenue, is probably not going to add in analytic features for Skip just for the sake of some third party.