I don't think it will happen from one or two carriers. They might block Russia's traffic on their own network, but based on
the submarine cable map they'll need a whole bunch of countries to get in on it to cut off Russia completely. Otherwise routes will update, and sure, traffic might slow down a bit, but isolation is very optimistic unless Russia chooses to isolate themselves.
The article even claims Cogent is blocking traffic, but I suspect it might be blocking particular IPs/domains since I did a visual traceroute just now to Yandex and it showed it was going through Cogent links. Either that the block was a very temporary measure.