Earlier this week the Kremlin directed state-owned Russian websites to switch from foreign hosting services to Russian-owned ones and to remove any Javascript created by foreign companies. Do you think this action is intended primarily to protect Russian state-owned sites from cyberattack, or do you think this is the first step in Russia severing ties with the global internet?

Russia is trying to remove any weak points, like having a foreign CDN in their websites, or hosting it overseas.
They are to create a self sustainable and closed economy, just like the cold war era.

I think Russia is trying to transition its economy to be more self-sustainable and not to rely as much on the western technology so any future sanctions will have lesser effect

It's both. I would not at all be surprised to see Russia ban individual contributions to open source communities or projects as a next step.

It has a dual use.  First, it cuts off the Russian population from learning the real truth about what is going on, second, it will mitigate any attacks on Russia from outside parties who may want to harm the Russian infrastructure, or backdoor their way into military resources and targets.

It is to be expected, I'd even think it caught them unprepared and now they are trying to cut the dependency from the rest of the world on a hurry, so errors are to be expected too.