You got the timeline wrong.

The break-up of Yugoslavia was a long, arguably still on-going, process, the final phase of which happened peacefully. Serbia and Montenegro, that made the post-1992 Yugoslavia, agreed in 2003 to change the name of the country to Serbia and Montenegro, pending the Montenegrin independence referendum scheduled for 2006.

Considering the possibility of another country name depreciation in three years, they agreed to keep the yu domain.

Fun fact, had the Montenegrin referendum gone the other way, the plan was to use .cs as the national domain, which used to be owned by another ex-country, Czechoslovakia.