Only select banks were banned from SWIFT, so they can still use or access Euros and Dollars.
Germany is currently still paying in Euros and Dollars for Russian Gazprom imports.
That selected banks include Russian Central bank, doesn't it?
Foreign reserve's purpose is that when a Russian company need to import anything like phone or cloth, it gives its rubel to commercial bank and gets foreign currency, let's say Euro. Then the company uses Euro to import from foreign market. The commercial bank in turn gets Euro from the Russian central bank when it runs out of Euro.
Even though that commercial bank is not cut off from SWIFT, there is a huge shortage of EURO in the whole Russian Forex reserve. When these commercial banks run out of their Euro, where would they get the needed Euro with Rubel? Certainly not through SWIFT from Europe, but Russian Central bank.
Therefor, sanction on Russian Central bank and freezing its Euro asset is a sanction on the whole Russian financial system and the whole Russian population regardless who is on the sanction list. This is exactly what the western sanction on Saddam Hussain did to the whole Iraqi population. "Targeted sanction" is only a cover up to fool the wold populace and for the westerners to pretend innocence.
So the only sensible thing for Russia to do is to break that sanction by demanding Rubel payment, or cut off the oil and gas to Europe.