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I am allowed to splice up my personal copies of videos.
But in this case you don't have one in the first place.
Legally able to watch and legally able to splice up are at the same level, as far as copyright is concerned. And I don't even need to make an extra copy to do the kind of live splicing an ad blocker does.
A data point is TiVo who are, apparently, still around and have a 'skip ads' button on recorded content.