“deep into russian territory” is quite an exaggeration. Biden only okayed it for the Kursk and neighbouring regions.

The U.S. official, who wasn’t authorized to speak publicly about the decision, said the U.S. is allowing Ukraine to use the weapons to target in and around Kursk — the same region where some 10,000 North Korean troops were recently deployed, according to the U.S. and its allies.

Source: NPR