This isn't a conspiracy theory; it's exactly what the article is arguing. Now that net neutrality might hurt Netflix rather than benefit them, not only are they not motivated to advocate for it (which, as the above comment pointed out, would not require any significant effort), but they are probably more inclined to advocate against it.

This is might be seen as an instance of "pulling the ladder up behind you".

They went through a lot of effort to make e.g. Verizon look bad if they didn't oppose NN.

Lots of back-and-forth in 2014, but here are major HN discussions:

https://news.ycombinator.com/item?id=7872168

https://news.ycombinator.com/item?id=8016055

That's not really a conspiracy theory, other than the current lack of evidence. That is just business as usual in most areas of business. It is just rare on the Internet, for reasons I don't completely understand.