Apart from the brief period from mid nineties until 2005 there were always gatekeepers who owned the presses and distribution chains. What changed?

This 50+ page document states many things, among which the claim "suddenly there are gatekeepers who own presses and distribution chains" is nowhere to be found.

Well the title explicitly says "New Gatekeepers".

Yes, but it doesn't say gatekeeping is only when newspapers are privately owned. It's a bit like person A saying they have a new car, person B conflating "car" with "any means of transportation", and then acting puzzled because person A doesn't have an airplane.

Which is not mutually exclusive with there having been Old Gatekeepers.

Strictly yes, but the title implies that something changed which is exactly what upthread commenter asked about.

Well, I would say - having grown up in the early days of the internet - that before the 'monopoly model' of internet companies, consumers simply didn't tolerate gatekeeping by private companies. It used to be easier to find the dissenting voices online than it is today, and keeping out minority opinions was seen as delivering an inferior product.

Essentially, big tech had been in a Moloch trap to the benefit of consumers, and now they've managed to co-ordinate their way out of it.

I see plenty of "dissenting voices" online, they're just not very worthwhile. I assume that a lot of what used to stand out as "correct" contrarianism has since become mainstream, or close enough.

Dissenting voices are only worthwhile if they agree with my worldview. My contrarian opinion is the correct one and has since won out.