Much of the soil in Russia that will warm is indeed acidic and largely inexploitable, which I suspect is what you were getting at; to their credit though, they picked an area that isn't (IIRC). Not that it changes much.

Most soil can be used for some type of farming with some help from fertilizers. Temperature and access to water is more critical. Higher CO2 also increases crops, in isolation.

We're also talking about a time span of 100-300 years. Keep in mind how much more efficient farming is now compared to even the 1950's.