The term "spoilers" usually refer to small but impactful parts of the plot (character X dies). If this is a translation perhaps it's referring to something else. The post says the anime content was condensed - I take as containing the most impactful scenes (perhaps even the majority of the work). Perhaps being a market substitution "spoils" it from being re-watched from 1st party source.

Copyright law exists so creators can create works and bring them to market in a economical manner.

If I livestream me reading a mystery novel in it's entirety I've devalued that work. This is because any viewers no longer could receive the same enjoyment from the novel meaning they're less likely to buy it. I've essentially republished their work, violating their copyright.