It could still be argued as net neutrality, because the browser with the largest market share is slowing down bits on their way to a different browser when it comes to their video service.
It also should be viewed negatively through an anti-competitive/monopolization lens.
If the internet is truly and open platform where no bits are treated differently on the path to the user based on their content, then this is inherently antithetical to that. Slowing down bits because you don't like whats in them or where they are going is fundamentally breaking Net Neutrality rules. The interruption of bits on their path is what makes it a Net Neutrality issue.