This is referring to a "crypto-ruble" - the article is talking about some sort of permissioned blockchain which could theoretically be issued by governments, at which point it would be fairly similar to existing digital fiat money.
This is essentially fiat currency that we're talking about, and it would not in any way prevent people who are interested in decentralisation from continuing to use a permissionless blockchain like BTC to preserve their freedom.
If they could apply tax to our fiat currency income
Just to clarify - a lot of people illegally evade tax by accepting cash, especially the self-employed. Governments are not always capable of applying all of the appropriate tax to physical cash, but a move to government-issued digital money
could probably be controlled well enough for that. It's important to distinguish between government money and our current perception of cryptocurrencies.