> But the journalists present this story as Russia's voluntary step towards isolating itself from the global internet. I am not sure why.

Because sanctioning a country off the internet has never been done. It's not a credible threat given the topology. Russia is land adjacent to many countries that would be unlikely to participate in a partitioning led by the US, and such a partition would not be very effective if only some countries participated.

Neither North Korea nor Iran are or have been disconnected from the global internet or phone systems by outside countries, and they are the most sanctioned nations.