Maybe it was just an excuse for the US to refuse the request from Kyiv because they were aware that supplying long-range weapons that could hit Russian territory would cause a third world war. Although the US and Nato are helping Ukraine a lot, they also do not want a nuclear clash to break out because that is the beginning of World War 3. The US does not want the war to end, but neither does it want it to become a world war.
This is not the first time the US has refused a request from Ukraine when it asked for the supply of heavy, long-range weapons.
If you study Russian history, you will find that it was the US and the wealthy elite that put Russia into power through the Bolshevik Revolution. Few Americans understand this.
People aren't stupid; certainly not Russians. Once Russia got the idea of the strength of technology into their heads, they became powerful... by being given some by the US, then by stealing some of it from other countries, and by inventing some of it on their own.
The US doesn't want to give Ukrainians too much advanced armament. Why not? Because they saw how giving too much to the Russians backfired on them.
Russia and Ukraine are having a bit of a squabble right now. But they are essentially people from the same part of the world... often brothers and sisters. Some aspects of the Ukrainian war are similar to a civil war. Once the squabble is over, who will the Ukrainians side with? The US or Russia?
A further danger to the US is, if they don't do it right, much of Nato might switch sides, and turn against the US sometime after the Ukraine war. There are lots of things to think about. Remember, in times of early WW2 or before, Stalin and Hitler were buddies to some extent. Some of Nato aren't too happy with what the US is doing right now.