Yup. You are right. In this case the US Treasury has frozen the account of the the Central Bank of the Russian Federation(CBR) holds with it. So the CBR cannot transact in USD, and in turn any Russian Bank relying on the CBR providing dollars for purchases of foreign goods and services will not be able transact through that route. It is not exclusion from SWIFT itself that is causing difficulties for Russia in this case, but freezing of the CBRs account with the Fed.

However, transacting through the route that goes from consumer -> Russian Bank -> CBR -> Fed -> US Bank -> US merchant (or in the reverse direction) is not the only route. A Russian bank could also transact with a correspondent bank in another country that transacts with a US bank that transacts with the Fed. So the following transaction might still be possible: consumer -> VTB (Russian bank) -> ABN Amro (dutch bank) -> Citi (US bank) -> Fed -> Bank of America -> US merchant. In fact this is probably the more common path (except VTB might have directly transacted with Citi before last week).

The easiest way to stop these other routes is to ban all Russian Banks from the SWIFT system. This is because pretty much every US bank uses SWIFT to serve as the messaging platform by which they transact with correspondent banks (And they will NOT use any other system -- because of US regulations). So simply preventing SWIFT messaging for all Russian banks makes it enormously difficult for Russian Banks to transact in USD.

Hence the emphasis on blocking SWIFT.

But it is not the end of the world for Russia. They still have three avenues for international trade:

1. Trade in another currency, preferably of a country that is less susceptible to US pressure (e.g. China, and eventually their own) This avoids SWIFT, the US Fed and US controllable financial entities altogether.

2. Use proxies. Have an entity located in another country trade in USD on behalf of a Russian entity. It is usual to keep such proxy relationships secret. However, should US govt. discover the proxy aiding a sanctioned entity, they may sanction the proxy itself, and the jurisdiction in which the proxy does business. So this method of transacting is risky for many entities (both banks and other non-financial entities.) So most won't do it. But some will.

3. Barter. This is not as ridiculous as it sounds. For example India had to use this system to buy critically needed oil from Iran, initially, after the latter was banned from SWIFT. For a very large country like India or (especially) China, that domestically produce a very wide range of goods and services, it can be very viable way to trade. But it is still limiting. Both countries have since moved to their own SWIFT like systems to trade with Iran. But of course, one cannot buy US produced (or more generally western goods) this way. So no iPhones.